[5.4] Fix a case where causedByLostConnection is passed a throwable

[themsaid]

In https://github.com/laravel/framework/blob/5.4/src/Illuminate/Database/DetectsLostConnections.php#L16 causedByLostConnection() receives the exception as the first argument but it's type hinted Exception, in this case if a throwable was thrown instead we get something like:

Argument 1 passed to Illuminate\Queue\Worker::causedByLostConnection() must be an instance of Exception, instance of Error given

This PR fixes that.