return NotFoundError if room not found

Per the Client-Server API[0] we should return
`M_NOT_FOUND` if the room isn't found instead
of generic SynapseError.

This ensures that /directory/list API returns
404 for room not found instead of 400.

[0]: https://matrix.org/docs/spec/client_server/unstable.html#get-matrix-client-r0-directory-list-room-roomid

Signed-off-by: Serban Constantin <serban.constantin@gmail.com>

AUTHORS.rst

@@ -62,4 +62,7 @@ Christoph Witzany <christoph at web.crofting.com>
  * Add LDAP support for authentication
 
 Pierre Jaury <pierre at jaury.eu>
-* Docker packaging
+* Docker packaging
+
+Serban Constantin <serban.constantin at gmail dot com>
+ * Small bug fix

changelog.d/2952.bugfix

@@ -0,0 +1 @@
+/directory/list API returns 404 for room not found instead of 400

synapse/rest/client/v1/directory.py

@@ -18,7 +18,7 @@ import logging
 
 from twisted.internet import defer
 
-from synapse.api.errors import AuthError, Codes, SynapseError
+from synapse.api.errors import AuthError, Codes, NotFoundError, SynapseError
 from synapse.http.servlet import parse_json_object_from_request
 from synapse.types import RoomAlias
 
@@ -159,7 +159,7 @@ class ClientDirectoryListServer(ClientV1RestServlet):
     def on_GET(self, request, room_id):
         room = yield self.store.get_room(room_id)
         if room is None:
-            raise SynapseError(400, "Unknown room")
+            raise NotFoundError("Unknown room")
 
         defer.returnValue((200, {
             "visibility": "public" if room["is_public"] else "private"