Imporve error message when MusicBrainz is unreachable

### Problem

Running `beet import ...` when MusicBrainz is down (as it's been quite a bit lately) throws an exception that should probably be caught and printed as a more readable error message:

```
musicbrainz: Error searching MusicBrainz: HTTPSConnectionPool(host='musicbrainz.org', port=443): Max retries exceeded with url: /ws/2/release?limit=10&query=release%3A%28haniwa%29+artist%3A%28kiyohiko+senba+and+his+haniwa+all+stars%29+date%3A%281983%29+catno%3A%28srcl2133%29&fmt=json (Caused by SSLError(SSLEOFError(8, '[SSL: UNEXPECTED_EOF_WHILE_READING] EOF occurred in violation of protocol (_ssl.c:1082)')))
Traceback (most recent call last):
File "/usr/lib/python3.14/site-packages/urllib3/connectionpool.py", line 464, in _make_request
self._validate_conn(conn)
~~~~~~~~~~~~~~~~~~~^^^^^^
File "/usr/lib/python3.14/site-packages/urllib3/connectionpool.py", line 1106, in _validate_conn
conn.connect()
~~~~~~~~~~~~^^
File "/usr/lib/python3.14/site-packages/urllib3/connection.py", line 796, in connect
sock_and_verified = _ssl_wrap_socket_and_match_hostname(
sock=sock,
...<14 lines>...
assert_fingerprint=self.assert_fingerprint,
)
File "/usr/lib/python3.14/site-packages/urllib3/connection.py", line 975, in _ssl_wrap_socket_and_match_hostname
ssl_sock = ssl_wrap_socket(
sock=sock,
...<8 lines>...
tls_in_tls=tls_in_tls,
)
File "/usr/lib/python3.14/site-packages/urllib3/util/ssl_.py", line 433, in ssl_wrap_socket
ssl_sock = _ssl_wrap_socket_impl(sock, context, tls_in_tls, server_hostname)
File "/usr/lib/python3.14/site-packages/urllib3/util/ssl_.py", line 477, in _ssl_wrap_socket_impl
return ssl_context.wrap_socket(sock, server_hostname=server_hostname)
~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.14/ssl.py", line 455, in wrap_socket
return self.sslsocket_class._create(
~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
sock=sock,
^^^^^^^^^^
...<5 lines>...
session=session
^^^^^^^^^^^^^^^
)
^
File "/usr/lib/python3.14/ssl.py", line 1076, in _create
self.do_handshake()
~~~~~~~~~~~~~~~~~^^
File "/usr/lib/python3.14/ssl.py", line 1372, in do_handshake
self._sslobj.do_handshake()
~~~~~~~~~~~~~~~~~~~~~~~~~^^
ssl.SSLEOFError: [SSL: UNEXPECTED_EOF_WHILE_READING] EOF occurred in violation of protocol (_ssl.c:1082)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/lib/python3.14/site-packages/urllib3/connectionpool.py", line 788, in urlopen
response = self._make_request(
conn,
...<10 lines>...
**response_kw,
)
File "/usr/lib/python3.14/site-packages/urllib3/connectionpool.py", line 488, in _make_request
raise new_e
urllib3.exceptions.SSLError: [SSL: UNEXPECTED_EOF_WHILE_READING] EOF occurred in violation of protocol (_ssl.c:1082)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/usr/lib/python3.14/site-packages/requests/adapters.py", line 696, in send
resp = conn.urlopen(
method=request.method,
...<9 lines>...
chunked=chunked,
)
File "/usr/lib/python3.14/site-packages/urllib3/connectionpool.py", line 872, in urlopen
return self.urlopen(
~~~~~~~~~~~~^
method,
^^^^^^^
...<13 lines>...
**response_kw,
^^^^^^^^^^^^^^
)
^
File "/usr/lib/python3.14/site-packages/urllib3/connectionpool.py", line 872, in urlopen
return self.urlopen(
~~~~~~~~~~~~^
method,
^^^^^^^
...<13 lines>...
**response_kw,
^^^^^^^^^^^^^^
)
^
File "/usr/lib/python3.14/site-packages/urllib3/connectionpool.py", line 872, in urlopen
return self.urlopen(
~~~~~~~~~~~~^
method,
^^^^^^^
...<13 lines>...
**response_kw,
^^^^^^^^^^^^^^
)
^
[Previous line repeated 3 more times]
File "/usr/lib/python3.14/site-packages/urllib3/connectionpool.py", line 842, in urlopen
retries = retries.increment(
method, url, error=new_e, _pool=self, _stacktrace=sys.exc_info()[2]
)
File "/usr/lib/python3.14/site-packages/urllib3/util/retry.py", line 543, in increment
raise MaxRetryError(_pool, url, reason) from reason # type: ignore[arg-type]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='musicbrainz.org', port=443): Max retries exceeded with url: /ws/2/release?limit=10&query=release%3A%28haniwa%29+artist%3A%28kiyohiko+senba+and+his+haniwa+all+stars%29+date%3A%281983%29+catno%3A%28srcl2133%29&fmt=json (Caused by SSLError(SSLEOFError(8, '[SSL: UNEXPECTED_EOF_WHILE_READING] EOF occurred in violation of protocol (_ssl.c:1082)')))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/lib/python3.14/site-packages/beets/metadata_plugins.py", line 420, in _search_api
response_data = self.get_search_response(params)
File "/usr/lib/python3.14/site-packages/beetsplug/musicbrainz.py", line 761, in get_search_response
return self.mb_api.search(
~~~~~~~~~~~~~~~~~~^
mb_entity, dict(params.filters), limit=params.limit
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "/usr/lib/python3.14/site-packages/beetsplug/_utils/musicbrainz.py", line 685, in search
return self._get_resource(entity, **kwargs)[f"{normalised_entity}s"]
~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.14/site-packages/beetsplug/_utils/musicbrainz.py", line 637, in _get_resource
return self.get_json(f"{self.api_root}/{resource}", params=kwargs)
~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.14/site-packages/beetsplug/_utils/musicbrainz.py", line 623, in get_json
return self._normalize_data(super().get_json(*args, **kwargs))
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.14/site-packages/beetsplug/_utils/requests.py", line 227, in get_json
return self.get(*args, **kwargs).json()
~~~~~~~~^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.14/site-packages/beetsplug/_utils/requests.py", line 215, in get
return self.request("get", *args, **kwargs)
~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.14/site-packages/beetsplug/_utils/musicbrainz.py", line 619, in request
return super().request(*args, **kwargs)
~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.14/site-packages/beetsplug/_utils/requests.py", line 211, in request
return self.session.request(*args, **kwargs)
~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.14/site-packages/beetsplug/_utils/requests.py", line 100, in request
r = super().request(*args, **kwargs)
File "/usr/lib/python3.14/site-packages/requests/sessions.py", line 651, in request
resp = self.send(prep, **send_kwargs)
File "/usr/lib/python3.14/site-packages/requests_ratelimiter/requests_ratelimiter.py", line 104, in send
response = super().send(request, **kwargs)
File "/usr/lib/python3.14/site-packages/requests/sessions.py", line 784, in send
r = adapter.send(request, **kwargs)
File "/usr/lib/python3.14/site-packages/beetsplug/_utils/requests.py", line 135, in send
return super().send(request, *args, **kwargs)
~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.14/site-packages/requests/adapters.py", line 727, in send
raise SSLError(e, request=request)
requests.exceptions.SSLError: HTTPSConnectionPool(host='musicbrainz.org', port=443): Max retries exceeded with url: /ws/2/release?limit=10&query=release%3A%28haniwa%29+artist%3A%28kiyohiko+senba+and+his+haniwa+all+stars%29+date%3A%281983%29+catno%3A%28srcl2133%29&fmt=json (Caused by SSLError(SSLEOFError(8, '[SSL: UNEXPECTED_EOF_WHILE_READING] EOF occurred in violation of protocol (_ssl.c:1082)')))
```

### Setup

* Python version: 3.14.7
* beets version: 2.13.1

Start at beetsplug/musicbrainz.py:761 and follow the request path through beetsplug/_utils/musicbrainz.py and beetsplug/_utils/requests.py, then reproduce the failure with beet import. Trace how _search_api handles the request exception; done means an unreachable MusicBrainz service produces a readable error instead of the shown traceback.

Written by the indexing model from the issue text.