These are the same that the one in chromium's src/PRESUBMIT.py but slightly more
resilitent to exceptions.
Empirical studies have shown that exceptions in presubmit checks result in
disorientation and lack of willingness to commit.

TEST=OMG new unit tests!

Review URL: http://codereview.chromium.org/1119003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@42102 0039d316-1c4b-4281-b951-d872f2087c98