Earlier the behavior was to raise an exception thereby stopping the
whole sync. Now we log an error message and skip the field. Also
fixes the `query_ldap` command to report missing fields without
error.
Fixes: #11780.
This should make life a lot more convenient for organizations that use
the LDAP integration and have their avatars in LDAP already.
This hasn't been end-to-end tested against LDAP yet, so there may be
some minor revisions, but fundamentally, it works, has automated
tests, and should be easy to maintain.
Fixes#286.