Fixed PHP-1086: Inserting documents with WriteConcern during failover broken

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
Merge pull request #691 from bjori/PHP-1086-failure-with-w1-should-throw-exception

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

* bjori/PHP-1086-failure-with-w1-should-throw-exception:
  Fixed PHP-1086: Inserting documents with WriteConcern during failover broken