Adds ability to propogate exceptions within map

Adds init variable for extension managers to specify whether
or not exceptions are propated up by map or are logged and supressed
Formatting fix

Fixes too many blank lines in test_extension.py
Update docstrings for map exception propagation

Updates docstrings related to map exception propagation
and the history.rst file
Merge pull request #22 from cyeoh/propogate_exceptions

Propogate exceptions