SAML2 Expired SAML2 metadata gives cryptic error.

This issue has been migrated from [#8721](https://github.com/matrix-org/synapse/issues/8721).

---

A host's saml2 metadata recently expired (as seen by the `validUntil` attribute on the metadata):
```

```
When trying to login, the user is sent to the `https://host/_matrix/client/r0/login/sso/redirect` page and shown a `M_UNKNOWN` ISE page rather than a useful error.

Digging into the logs, the error given by Synapse is also not terribly helpful:

```
2020-11-04 @ 22:55:45.487 saml2.client_base.IdpUnspecified: No IdP to send to given the premises
2020-11-04 @ 22:55:45.487 raise IdpUnspecified("No IdP to send to given the premises")
2020-11-04 @ 22:55:45.487 File "/usr/local/lib/python3.7/site-packages/saml2/client_base.py", line 237, in _sso_location
2020-11-04 @ 22:55:45.487 destination = self._sso_location(entityid, binding)
2020-11-04 @ 22:55:45.487 File "/usr/local/lib/python3.7/site-packages/saml2/client.py", line 115, in prepare_for_negotiated_authenticate
2020-11-04 @ 22:55:45.487 **kwargs)
2020-11-04 @ 22:55:45.487 File "/usr/local/lib/python3.7/site-packages/saml2/client.py", line 76, in prepare_for_authenticate
2020-11-04 @ 22:55:45.487 relay_state=client_redirect_url
2020-11-04 @ 22:55:45.487 File "/usr/local/lib/python3.7/site-packages/synapse/handlers/saml_handler.py", line 127, in handle_redirect_request
2020-11-04 @ 22:55:45.487 return self._saml_handler.handle_redirect_request(client_redirect_url)
2020-11-04 @ 22:55:45.487 File "/usr/local/lib/python3.7/site-packages/synapse/rest/client/v1/login.py", line 469, in get_sso_url
2020-11-04 @ 22:55:45.487 sso_url = await self.get_sso_url(request, client_redirect_url)
2020-11-04 @ 22:55:45.487 File "/usr/local/lib/python3.7/site-packages/synapse/rest/client/v1/login.py", line 402, in on_GET
2020-11-04 @ 22:55:45.487 callback_return = await raw_callback_return
2020-11-04 @ 22:55:45.487 File "/usr/local/lib/python3.7/site-packages/synapse/http/server.py", line 405, in _async_render
2020-11-04 @ 22:55:45.487 callback_return = await self._async_render(request)
2020-11-04 @ 22:55:45.487 File "/usr/local/lib/python3.7/site-packages/synapse/http/server.py", line 228, in _async_render_wrapper
2020-11-04 @ 22:55:45.487 Traceback (most recent call last):
2020-11-04 @ 22:55:45.487
2020-11-04 @ 22:55:45.487 During handling of the above exception, another exception occurred:
2020-11-04 @ 22:55:45.487
2020-11-04 @ 22:55:45.487 IndexError: list index out of range
2020-11-04 @ 22:55:45.487 srvs = self.metadata.single_sign_on_service(list(eids.keys())[0], binding)
2020-11-04 @ 22:55:45.487 File "/usr/local/lib/python3.7/site-packages/saml2/client_base.py", line 234, in _sso_location
2020-11-04 @ 22:55:45.487 Traceback (most recent call last):
2020-11-04 @ 22:55:45.487 2020-11-04 22:55:45,486 - synapse.http.server - 83 - ERROR - GET-176132- Failed handle request via 'SAMLRedirectServlet'
```

What would be good here is some kind of error that tells you to update your metadata file. Ideally this would be communicated to the user.