Introduce Two Levels of Errors

[aaron97neu]

It was suggested during the 11/30 research meeting to implement 2 levels of errors, one for app level errors and another for internal bugs. This would help the programmer distinguish from errors in his program or internal errors that he cannot fix (and should then report). One approach for this could be to create two custom exception classes in Python, or use two of the built in exceptions found here (https://docs.python.org/2/library/exceptions.html)