repopick: Allow the github fetch to fail

* This is optional and done to save gerrit server
  bandwidth, however it may fail in cases where
  the 'github' remote is a mirror which doesn't
  sync the changes.
* Let it try fetching from gerrit if fetching from
  github fails.

Change-Id: I6d183ff83572d817d78633280d8b20e3efdaf8f0