implement Exception message methods to avoid DepreciationWarning
implement Exception message methods to avoid DepreciationWarning

Extending Exception and using a variable called self.message raises the following warning:
totalimpact/providers/provider.py:183: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
Address by adding message property as suggested in http://stackoverflow.com/questions/1272138/baseexception-message-deprecated-in-python-2-6

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