fix(on-move): ZMSA-73: on move if failed to get folder write lock, treat as tempfail (#970)

* ZMSA-73: on move if failed to get folder write lock, return error message as string instead of object

* regard write lock error as tempfail

Author: NickOvt

lib/handlers/on-move.js

@@ -20,7 +20,7 @@ module.exports = (server, messageHandler) => (mailbox, update, session, callback
         }
 
         if (!lock.success) {
-            return callback(null, new Error('Failed to get folder write lock'));
+            return callback(new Error('Failed to get folder write lock'));
         }
 
         messageHandler.move(