rewrite pullObject

[simonLeary42]

While writing tests for disbanding and recreating PI groups, I found that an LDAPEntry still thinks it exists after being deleted. This is because $this->object is not set back to null in pullObject.

rewrote pullObject:

• set $this->object to null if ldap_read or ldap_get_entries return false
• removed comment about "LDAP count element" because it's removed with LDAP::stripCount
• renamed $search to $entries
• replaced check for isset($entries) with $entries === false
• replaced die() with throw new Exception
• returned true on success