authn.configuration: Improve error handling in openid_configuration()

@victorlin suggested¹ we improve error handling in this code path, and
he's right.

Catch and improve the output for the three most common errors we'll
encounter: connection errors, HTTP errors, and JSON decoding errors.
The original error is summarized in the error output, and by chaining
the new exception from the original, the full chain in all its detail
will be printed when NEXTSTRAIN_DEBUG=1 for troubleshooting.

¹ <#333 (comment)>

Author: tsibley

nextstrain/cli/authn/configuration.py

@@ -20,18 +20,31 @@ def openid_configuration(origin: Origin):
     assert origin
 
     with requests.Session() as http:
-        response = http.get(origin.rstrip("/") + "/.well-known/openid-configuration")
+        try:
+            response = http.get(origin.rstrip("/") + "/.well-known/openid-configuration")
+            response.raise_for_status()
+            return response.json()
 
-        if response.status_code == 404:
+        except requests.exceptions.ConnectionError as err:
             raise UserError(f"""
-                Failed to retrieve authentication metadata for {origin}.
+                Could not connect to {origin} to retrieve
+                authentication metadata:
+
+                    {type(err).__name__}: {err}
+
+                That remote may be invalid or you may be experiencing network
+                connectivity issues.
+                """) from err
+
+        except (requests.exceptions.HTTPError, requests.exceptions.JSONDecodeError) as err:
+            raise UserError(f"""
+                Failed to retrieve authentication metadata for {origin}:
+
+                    {type(err).__name__}: {err}
 
                 That remote seems unlikely to be an alternate nextstrain.org
                 instance or an internal Nextstrain Groups Server instance.
-                """)
-
-        response.raise_for_status()
-        return response.json()
+                """) from err
 
 
 def client_configuration(origin: Origin):