Bulk find() requires selector, SERVER-12553.

Squashed:
- Update comments in bulk_api.js.
- Throw Error from invalid Bulk operations, not strings

Signed-off-by: Matt Kangas <matt.kangas@mongodb.com>
SERVER-12654 append additional information to inconsistent config writes

Signed-off-by: Matt Kangas <matt.kangas@mongodb.com>