protect against bad schemas

This addresses part 3 of #58.

* Avoid crashing `yz_events` server during index creation.
* Make best effort to create index, log on error and retry periodically.
* Add test to verify bad schema can be uploaded and corrected without bringing the cluster down.
* Remove catch from unit test to avoid masking failure.
* Fix spec.
* Handle error from `yz_solr:cores` during tick event.