narrowsReducer/eventUpdateMessageFlags: Assert operation is add | remove

Adds an assertion to match the type in EventUpdateMessageFlagsAction
in src/actionTypes.js, so that action.operation can only be 'add' or
'remove'.

src/chat/narrowsReducer.js

@@ -74,6 +74,8 @@ const eventUpdateMessageFlags = (state, action) => {
         updates.push({
           [narrowStr]: state[narrowStr].filter(id => !messagesSet.has(id)),
         });
+      } else {
+        throw new Error(`Unexpected operation ${operation} in an EVENT_UPDATE_MESSAGE_FLAGS action`);
       }
     }