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/directory.py

@@ -18,7 +18,13 @@ import logging
 
 from twisted.internet import defer
 
-from synapse.api.errors import AuthError, Codes, NotFoundError, SynapseError
+from synapse.api.errors import (
+    AuthError,
+    Codes,
+    InvalidClientCredentialsError,
+    NotFoundError,
+    SynapseError,
+)
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
 from synapse.rest.client.v2_alpha._base import client_patterns
 from synapse.types import RoomAlias
@@ -97,7 +103,7 @@ class ClientDirectoryServer(RestServlet):
                 room_alias.to_string(),
             )
             defer.returnValue((200, {}))
-        except AuthError:
+        except InvalidClientCredentialsError:
             # fallback to default user behaviour if they aren't an AS
             pass