This modification make gclient-new-workdir.py display explicit error messages instead of cryptic Python backtraces.

BUG=

Review URL: https://codereview.chromium.org/68213010

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@235408 0039d316-1c4b-4281-b951-d872f2087c98