Exception caught is assumed to have strerror attribute

[Brad Crittenden]

When running charm-proof -b on something that is not recognized to be a bundle, the exception is raised but then not processed by an error in the exception handling when e.strerror does not exist.

An absurd example

[trunk]bac@trusty64:~/projects/juju-quickstart$ charm-proof -b ~/.emacs
Traceback (most recent call last):
  File "/usr/bin/charm-proof", line 9, in <module>
    load_entry_point('charm-tools==1.2.8', 'console_scripts', 'charm-proof')()
  File "/usr/lib/python2.7/dist-packages/charmtools/proof.py", line 63, in main
    args.debug)
  File "/usr/lib/python2.7/dist-packages/charmtools/proof.py", line 54, in proof
    return ["FATAL: %s" % e.strerror], 200
AttributeError: 'exceptions.Exception' object has no attribute 'strerror'

It was raising the 'Not a bundle' exception, which can be found in e.message.