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

`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.
