updated logger message

* the error is not specific to multi_plugin, and can happen with normal plugins
* logging the line that caused this error and the plugin name
better handling of intervals

* reducing the actual processing time from the interval, so it runs
  in more accurate intervals independent of the processing time.
* if processing time takes longer than the interval, we run it instantly
performance - reducing carbon footprint :)

* Establishing one carbon connection for all plugins, instead of one connection
  for each. This rather drastically improves performance. From a few measurements
  this reduces processing time by over 50%
* This can be even further enhanced, to keep one persistent carbon (/munin?)
  connection and only re-establish it if it drops (For Future)
Merge pull request #2 from gingerlime/master

updated logger message