Fixed bug in DropController.retrieve() where a special state led to a drop message be handled twice or more times. #200
Added rationale comment to DropController.retrieve() why InvalidKeyExceptions won't be handled
Pass DropController.sendAndForget() exception to logger for better stacktrace analysis
Pass exception in DropController.handleDrop() to logger
Merge pull request #212 from aBothe/pr

Fixed DropController.retrieve()-Bug; Improved exception logging