[Fix] Format exception to string in failed jobs

[divine]

Format exception to string in failed job so it can be saved correctly.

See: https://github.com/laravel/framework/blob/6.x/src/Illuminate/Queue/Failed/DatabaseFailedJobProvider.php#L59

Closes: #1800, #1524, #1480

[divine]

We might actually do something like this:

    /**
     * Log a failed job into storage.
     * @param string $connection
     * @param string $queue
     * @param string $payload
     * @param  \Exception  $exception
     * @return \MongoDB\BSON\ObjectId|null
     */
    public function log($connection, $queue, $payload, $exception)
    {
        $failed_at = Carbon::now()->getTimestamp();

        $exception = (string) $exception;

        return $this->getTable()->insertGetId(compact(
            'connection', 'queue', 'payload', 'exception', 'failed_at'
        ));
    }

However insertGetId returns @inheritdoc which is int but in our case it's MongoDB\BSON\ObjectId

See:
https://github.com/jenssegers/laravel-mongodb/blob/master/src/Jenssegers/Mongodb/Eloquent/Builder.php#L66
and
https://github.com/jenssegers/laravel-mongodb/blob/master/src/Jenssegers/Mongodb/Query/Builder.php#L572

[Giacomo92]

Add tests, see here

[divine]

Add tests, see here

There is no complete tests for jobs so I'm not sure if we should do that now?

Also function log doesn't return insert id which should do that at first, then we could work out on adding good tests.

I think better example for tests would be https://github.com/laravel/framework/tree/6.x/tests/Queue

[Giacomo92]

Yes, Laravel tests are more complete. It would be awesome if you add tests for all job features.

[divine]

Yes, Laravel tests are more complete. It would be awesome if you add tests for all job features.

Even making a breaking changes is fine? 😀