[5.0] Log exceptions and their messages when a job fails.

Recreated this pull request for 5.0 instead of 4.2.

Fixes issue #6203.
Doesn't this mean you'll always see this exception instead of the one actually killing the job? 
That one is caught [here](https://github.com/laravel/framework/blob/4.2/src/Illuminate/Queue/Worker.php#L212-220) and it's not available when you call `$this->logFailedJob`. I believe you'd have to save the exception to a file(or somewhere else) for later use in this line.

Given a job can be released back to the queue when it fails, and that it can retry, it will have multiple exceptions as it 'reason' to fail, right? I believe those are the ones we should logging and showing in the `queue:failed`