Devise a better error handling / exception strategy

[mpreisler]

Right now we let the raw exceptions bubble through. The daemon is exception safe and cleans up in case of errors but the reporting of the errors is more suitable for developers than users.

@ybznek suggested that we should wrap common errors in exception classes specific to the daemon and show better messages.

Examples:
When task of ID X is not found we show "Task X was not found." instead of "File /a/b/c/d/tasks/1.xml cannot be opened".

This task includes the research of how to handle this.