Gracefully handle malformed DNs in LDAP group sync

Merged Drew Blessing requested to merge dblessing/gitlab:group_sync_dn_error_checking into master

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/23422

Customer reported an emergency because group sync was blowing up with 'Badly formed DN'. It turns out their LDAP system inserts a blank member entry at the beginning of each LDAP group. When we try to parse this DN, group sync explodes. Obviously we should handle this error gracefully. This merge request catches the error, logs it, and moves on (skipping that entry).

cc/ @jacobvosmaer-gitlab @rspeicher As we know now, it takes a village to properly write/review/ship LDAP code 😉 I appreciate your help.