Raise an exception in case an upstream branch can't be found.
Fix an exception that was occuring just before when no upstream is found.

Review URL: http://codereview.chromium.org/1755018

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@46011 0039d316-1c4b-4281-b951-d872f2087c98