Improve error if python can't run pep8

Summary:
On systems with an ancient version of python, the pep8 linter won't run.
Instead of blowing up in the user's face, we should display a nice error
message.

Test Plan:
Put /usr/bin (where the ancient version of python is) at the beginning of
my path and tried to lint some python. I got a nice error instead of a
stack trace.

Reviewers: epriestley, wez

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5356
Merge branch 'master' of http://github.com/facebook/arcanist