Call ChangeInfo.Load() with fail_on_not_found=True to get a good error message in this case.

BUG=none
TEST=manual

Review URL: http://codereview.chromium.org/8692007

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@112018 0039d316-1c4b-4281-b951-d872f2087c98