Add a check to see if the current query can/should be run using ES

[mboynes]

ES_WP_Query should have a check in place to see if the request can actually be executed successfully, and if not, defer to WP_Query.

For instance, if the ES index doesn't contain post_mime_type data, and the query includes it, that query should be executed against MySQL. Another example is a post__in query with no other arguments; ES would be of no benefit there.