Fix deprecation warning in setup.py

In Python 2.6, `setup.py` produced the following deprecation warning:

    setup.py:75: DeprecationWarning: BaseException.message has been
    deprecated as of Python 2.6

Changing `e.message` to `str(e)` fixes this.
Merge pull request #99 from lpsinger/fix_baseexception_deprecation_warning

Fix deprecation warning in setup.py