Plugin should log permission errors to logstash.log

[tomcashman]

I installed logstash 1.5 using the debian installer and set up a test conf to read apache logs and output to a file in /tmp/. However, nothing was being output to the file and no errors were appearing in /var/log/logstash/logstash.log

I discovered the problem was caused by logstash not having permission to read the apache log. I had to mess around with running logstash manually (rather than through init) and adding verbose logging, etc. and even still, there was no indication in the logs. This problem would have been a lot quicker for me to solve if there was an error in the logstash log saying that it had insufficient permissions to read the file.

[Crote]

Please note that, due to https://github.com/elastic/logstash/issues/2110, a large portion of users are GUARANTEED to have permission issues. This should be fixed ASAP.