server: Handle exception from group session sharing request.

matrix/server.py

@@ -878,10 +878,16 @@ class MatrixServer(object):
             self.handle_backlog_response(response)
 
         elif isinstance(response, KeysClaimResponse):
+            try:
                 _, request = self.client.share_group_session(
                     response.room_id,
                     ignore_missing_sessions=True
                 )
+            except OlmTrustError as e:
+                m = ("Untrusted devices found in room: {}".format(e))
+                self.error(m)
+                return
+
             self.send(request)
 
         elif isinstance(response, ShareGroupSessionResponse):