Finish implementing RM endpoint

- This change causes a 405 to be sent if "m.read_marker" is set via /account_data
 - This also fixes-up the RM endpoint so that it actually Works.

synapse/rest/client/v2_alpha/account_data.py

@@ -16,7 +16,7 @@
 from ._base import client_v2_patterns
 
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
-from synapse.api.errors import AuthError
+from synapse.api.errors import AuthError, SynapseError
 
 from twisted.internet import defer
 
@@ -82,6 +82,12 @@ class RoomAccountDataServlet(RestServlet):
 
         body = parse_json_object_from_request(request)
 
+        if account_data_type == "m.read_marker":
+            raise SynapseError(405,
+                "Cannot set m.read_marker through this API. "
+                "Use /rooms/!roomId:server.name/read_marker"
+            )
+
         max_id = yield self.store.add_account_data_to_room(
             user_id, room_id, account_data_type, body
         )