Fixed PHP-902: Segfault when unregistering broken server

This fixes a segfault in the mongocollection-find_error-001.phpt failover test

When the server doesn't work, we need to unregister it from the manager,
otherwise we keep a dangling pointer to it, and crash when we try to
locate servers to use.

Also fix invalid read, using the wrong error variable on error
Merge pull request #525 from bjori/PHP-902-uregister-broken-server

Fixed PHP-902: Segfault when unregistering broken server

* bjori/PHP-902-uregister-broken-server:
  Fixed PHP-902: Segfault when unregistering broken server