Fix format exception in failed jobs

Format exception to string so it can be saved correctly. See https://github.com/laravel/framework/blob/6.x/src/Illuminate/Queue/Failed/DatabaseFailedJobProvider.php#L59

src/Jenssegers/Mongodb/Queue/Failed/MongoFailedJobProvider.php

@@ -12,12 +12,15 @@ class MongoFailedJobProvider extends DatabaseFailedJobProvider
      * @param string $connection
      * @param string $queue
      * @param string $payload
+     * @param  \Exception  $exception
      * @return void
      */
     public function log($connection, $queue, $payload, $exception)
     {
         $failed_at = Carbon::now()->getTimestamp();
 
+        $exception = (string) $exception;
+
         $this->getTable()->insert(compact('connection', 'queue', 'payload', 'failed_at', 'exception'));
     }