Also, don't use assert for user error handling plus
some other general python cleanup.

NOPRESUBMIT=true (presubmit is broken)

Review URL: https://codereview.chromium.org/1091603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294950 0039d316-1c4b-4281-b951-d872f2087c98