Check for existence of refs upon initial fetch

When we do an initial fetch and have not specified any branch etc,
the following fetch command will not error:
git fetch origin --tags +refs/heads/*:refs/remotes/origin/*

In this change we make sure something got fetched and if not we report
an error.

This fixes the bug that occurs when we init using a bad manifest url and
then are unable to init again (because a manifest project has been
inited with no manifest).

Change-Id: I6f8aaefc83a1837beb10b1ac90bea96dc8e61156