Fixes and tests for issue 336

- Add web machine callback resource_available/2 to yz_wm_search, so that non-existent indexes in http queries are handled correctly.
- Change yz_pb_search:maybe_process to handle non-existent indexes and remove catch clause for same.
- Add a test for the http handling in yokozuna_essential
- Add a test for the PB handling in yz_pb

Issue:
https://github.com/basho/yokozuna/issues/336