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

project.py

@@ -1204,13 +1204,19 @@ class Project(object):
 
   def AbandonBranch(self, name):
     """Destroy a local topic branch.
+
+    Args:
+      name: The name of the branch to abandon.
+
+    Returns:
+      True if the abandon succeeded; False if it didn't; None if the branch
+      didn't exist.
     """
     rev = R_HEADS + name
     all = self.bare_ref.all
     if rev not in all:
-      # Doesn't exist; assume already abandoned.
-      #
-      return True
+      # Doesn't exist
+      return None
 
     head = self.work_git.GetHead()
     if head == rev: