[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.
The exceptions being thrown will be logged here: https://github.com/milesj/laravel-framework/blob/log-queue-errors-5.0/src/Illuminate/Queue/Worker.php#L220

Which is a few lines down from that line. That line throws a `RuntimeException` only when the max tries is hit.