sync: fix handling of -f and local checkouts

The partial clone rework (commit 745be2ed


"Add support for partial clone") changed the behavior when a single repo
hit a failure: it would always call sys.exit() immediately.  This isn't
even necessary as we already pass down an error event object which the
workers set and the parent checks.  Just delete the exit entirely.

Change-Id: Id72d8642aefa2bde24e1a438dbe102c3e3cabf48
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/233552
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>