stack trace when you have multiple saved servers and don't pick one

I have 3 saved/nicknamed servers. When deploying, I need to pick one using -n. When I forgot, instead of a helpful error message I got a stack trace:

❯ rsconnect deploy quarto index.qmd
Inspecting Quarto project...                     [OK]
Traceback (most recent call last):
  File "/Users/chris/content/quarto-email-install-fest/.venv/lib/python3.11/site-packages/rsconnect/main.py", line 97, in wrapper
    result = func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/chris/content/quarto-email-install-fest/.venv/lib/python3.11/site-packages/click/decorators.py", line 33, in new_func
    return f(get_current_context(), *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/chris/content/quarto-email-install-fest/.venv/lib/python3.11/site-packages/rsconnect/main.py", line 1219, in deploy_quarto
    ce = RSConnectExecutor(**kwargs)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/chris/content/quarto-email-install-fest/.venv/lib/python3.11/site-packages/rsconnect/api.py", line 424, in __init__
    self.setup_client(cookies)
  File "/Users/chris/content/quarto-email-install-fest/.venv/lib/python3.11/site-packages/rsconnect/api.py", line 540, in setup_client
    self.client = RSConnectClient(self.remote_server, cookies)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/chris/content/quarto-email-install-fest/.venv/lib/python3.11/site-packages/rsconnect/api.py", line 136, in __init__
    append_to_path(server.url, "__api__"),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/chris/content/quarto-email-install-fest/.venv/lib/python3.11/site-packages/rsconnect/http_support.py", line 129, in append_to_path
    if uri.endswith("/") and path.startswith("/"):
       ^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'endswith'
Internal error: 'NoneType' object has no attribute 'endswith'

related: #214

Start with the deploy_quarto entry point in rsconnect/main.py and follow RSConnectExecutor through rsconnect/api.py to the server selection and URL handling shown in the traceback. Reproduce the command with multiple saved servers and no -n; done means it reports a helpful selection error instead of a stack trace.