Fixed PHP-1086: Inserting documents with WriteConcern during failover br...

...oken

This is already covered by tests/replicaset-failover/mongocollection-insert-001.phpt
But as the failover testscases are so slow we only run them on Jenkins which is
why this wasn't discovered earlier

We don't actually support w=-1 (ignore socket errors) so we do want to throw exception
here in all cases