Exception Handling  #4

There are places which return an instance of 'Exception' which lacks any detail.

Node:
All calls to Node.GetNext() should raise an exception if Destination Nodes / Chooser were not set through Visits(...)

Visits(...) should raise an Exception if the nodes / Choosers given are Null

ProbabilityChooser
should raise an exception if all ItemWeights are set to 0

NodeEngine:
Start Method should raise an exception if the graphi is not valid