Lift Elasticsearch errors into something more explicit

@MichaelXavier I was hesitant to lift errors into a concrete datatype because they didn't document anything and I'd have to figure out how to reproduce everything for testing. Means having an "UnknownError" case is virtually unavoidable.

Seems a good idea to me.

Start by reviewing the issue discussion and locating the Elasticsearch error-handling code in the repository. Determine which errors are currently exposed and how they could be represented explicitly, including the proposed UnknownError case; done means the error model and its tests are defined and agreed upon.

Written by the indexing model from the issue text.