Active Directory auth: try other domain-controllers even when STARTTLS fails

When STARTTLS fails for a domain controller don't bail out: instead keep trying
the other domain controllers. E.g. it could fail for the same reason that
unencrypted binds could fail: server unavailability.

This change ensures that unavailable servers as well as servers with
unrecognized certificates are skipped while still logging a decent message for
server admins to find out what's going on.

Testing Done:
Tested this on my company's review board server. Without it I get a failure
every few login-attempts when one of our domain controllers is down (I've got
TLS enabled for active directory authentication). With this patch it only logs
a warning and goes on with the next server.

The fact that it only fails every once in a while is probably related due to
SRV order-randomisation within the same priority level (all of our domain
controllers get assigned the same priority).

Reviewed at https://reviews.reviewboard.org/r/5701/
Merge branch 'release-1.7.x' into release-2.0.x