Clarified debug output for incorrect syslog input, invalid syslog now ta...

Added helpful tag to clarify source of _grokparsefailure errors. This was after exhausting all docs and wasting a lot of time troubleshooting custom patterns, only to find that the problem was invalid syslog input to the internal grok filter used by the syslog input. I'm game for a different tagname if you don't like this one, but this would have saved me several hours if it had been in here a week ago :)