Clean up exception handling for access_tokens (#5656)

First of all, let's get rid of `TOKEN_NOT_FOUND_HTTP_STATUS`. It was a hack we
did at one point when it was possible to return either a 403 or a 401 if the
creds were missing. We always return a 401 in these cases now (thankfully), so
it's not needed.

Let's also stop abusing `AuthError` for these cases. Honestly they have nothing
that relates them to the other places that `AuthError` is used, other than the
fact that they are loosely under the 'Auth' banner. It makes no sense for them
to share exception classes.

Instead, let's add a couple of new exception classes: `InvalidClientTokenError`
and `MissingClientTokenError`, for the `M_UNKNOWN_TOKEN` and `M_MISSING_TOKEN`
cases respectively - and an `InvalidClientCredentialsError` base class for the
two of them.

synapse/rest/client/v1/room.py

@@ -24,7 +24,12 @@ from canonicaljson import json
 from twisted.internet import defer
 
 from synapse.api.constants import EventTypes, Membership
-from synapse.api.errors import AuthError, Codes, SynapseError
+from synapse.api.errors import (
+    AuthError,
+    Codes,
+    InvalidClientCredentialsError,
+    SynapseError,
+)
 from synapse.api.filtering import Filter
 from synapse.events.utils import format_event_for_client_v2
 from synapse.http.servlet import (
@@ -307,7 +312,7 @@ class PublicRoomListRestServlet(TransactionRestServlet):
 
         try:
             yield self.auth.get_user_by_req(request, allow_guest=True)
-        except AuthError as e:
+        except InvalidClientCredentialsError as e:
             # Option to allow servers to require auth when accessing
             # /publicRooms via CS API. This is especially helpful in private
             # federations.