Sync: Fix error exit code when both -n and -f are used

When repo sync is used with -f (--force-error) and a project fails to
sync, the sync will continue but then exit with an error status.

However if -n (--network-only) is also used, the exit code is 0, even
when a project failed.

Modify the logic to make sure the sync exits with the correct status.

Bug: Issue 214
Change-Id: I0b5d97a34642c5aa3743750ef14a42c9d5743c1d