This issue is fixed in Python 2.7.12, but we don't guarantee
that we use that version everywhere. The root issue is that,
on certain platforms (e.g. Mac), the exception indicating that
the underlying command (e.g. 'hg') was not found is too large,
and loading the pickle representing the exception fails.

This CL catches this specific exception and treats it identically
to the OS properly raising a 'command not found' code.

BUG=699173

Change-Id: I0483ab0d1d0b6a3dfcfd26f2d9644817b5b5284f
Reviewed-on: https://chromium-review.googlesource.com/455537Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>