sync: Use --force-broken to continue other projects

This adds a new flag -f/--force-broken that will allow the rest of
the sync process to continue instead of bailing when a particular
project fails to sync.

Change-Id: I23680f2ee7927410f7ed930b1d469424c9aa246e
Signed-off-by: Andrei Warkentin <andreiw@motorola.com>
Signed-off-by: Shawn O. Pearce <sop@google.com>