When a rebase generates a conflict, report an error and exit immediately.
If in an existing conflict (no branch), report an error and exit immediately.

Review URL: http://codereview.chromium.org/496003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@34459 0039d316-1c4b-4281-b951-d872f2087c98