fixed bug in pilot submission that caused the worker thread to crash in
case of a failed submission.

close #76

Andre: the error should be fixed. However the cause of it is the following piece
of code that tries to determine the 'home' directory. It's definetly a candidate
for the "ugly hacks top 3":

https://github.com/saga-project/saga-pilot/blob/master/src/sagapilot/mpworker/pilot_manager_worker.py#L250

If you have a better / smarter idea how to implement this, please let me know!