Fixed repo abandon to give better messages.

The main fix is to give an error message if nothing was actually
abandoned.  See <http://crosbug.com/6041>.

The secondary fix is to list projects where the abandon happened.
This could be done in a separate CL or dropped altogether if requested.

TEST=manual

$ repo abandon dougabc; echo $?
Abandon dougabc: 100% (127/127), done.
Abandoned in 2 project(s):
  chromite
  src/platform/init
0

$ repo abandon dougabc; echo $?
Abandon dougabc: 100% (127/127), done.
error: no project has branch dougabc
1

$ repo abandon dougabc; echo $?
Abandon dougabc: 100% (127/127), done.
error: chromite/: cannot abandon dougabc
1

Change-Id: I79520cc3279291acadc1a24ca34a761e9de04ed4
2 files changed