Extend error log API so that observer can be notified of new log entries

Fixes Slicer issue:
  http://www.na-mic.org/Bug/view.php?id=1349
  http://www.na-mic.org/Bug/view.php?id=2537
Merge branch 'tweak-error-log-api'

* tweak-error-log-api:
  Extend error log API so that observer can be notified of new log entries