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/api/errors.py

@@ -210,7 +210,9 @@ class NotFoundError(SynapseError):
 
 
 class AuthError(SynapseError):
-    """An error raised when there was a problem authorising an event."""
+    """An error raised when there was a problem authorising an event, and at various
+    other poorly-defined times.
+    """
 
     def __init__(self, *args, **kwargs):
         if "errcode" not in kwargs:
@@ -218,6 +220,35 @@ class AuthError(SynapseError):
         super(AuthError, self).__init__(*args, **kwargs)
 
 
+class InvalidClientCredentialsError(SynapseError):
+    """An error raised when there was a problem with the authorisation credentials
+    in a client request.
+
+    https://matrix.org/docs/spec/client_server/r0.5.0#using-access-tokens:
+
+    When credentials are required but missing or invalid, the HTTP call will
+    return with a status of 401 and the error code, M_MISSING_TOKEN or
+    M_UNKNOWN_TOKEN respectively.
+    """
+
+    def __init__(self, msg, errcode):
+        super().__init__(code=401, msg=msg, errcode=errcode)
+
+
+class MissingClientTokenError(InvalidClientCredentialsError):
+    """Raised when we couldn't find the access token in a request"""
+
+    def __init__(self, msg="Missing access token"):
+        super().__init__(msg=msg, errcode="M_MISSING_TOKEN")
+
+
+class InvalidClientTokenError(InvalidClientCredentialsError):
+    """Raised when we didn't understand the access token in a request"""
+
+    def __init__(self, msg="Unrecognised access token"):
+        super().__init__(msg=msg, errcode="M_UNKNOWN_TOKEN")
+
+
 class ResourceLimitError(SynapseError):
     """
     Any error raised when there is a problem with resource usage.