The recent changes to the chromite wrapper hid all import errors
in chromite, simply giving an error that chromite couldn't be found.
This fixes the problem.

For associated review in chromiumos git:
  http://codereview.chromium.org/6461009

Review URL: http://codereview.chromium.org/6487003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@74452 0039d316-1c4b-4281-b951-d872f2087c98