Have zuul handle merge failures.

If Zuul is unable to merge a change, don't run any jobs, and report
the merge failure to gerrit directly (but still observing the
dependent change queue, in case a change ahead caused the merge
failure).

Adds a test for this situation.

Change-Id: I1ee2a8846b159db385019352cc04af2140db81af
Reviewed-on: https://review.openstack.org/11421
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
4 files changed