Search: better error messages for failed queries

#### Is your feature request related to a problem? Please describe.
Right now when a user submits a query that has a bug in it so that the ES query fails, we only return a message that says: "query failed" and don't really give the user any info on why it failed. It creates a support burden for us and makes it harder for end users.

Improving this would probably also help our own devs when working and debugging issues.

#### Describe the solution you'd like
I'm not sure of the fix, but maybe we can take a guess at what is wrong with the query (I don't want us to return the error directly from ES because we have no control over what those error messages are). For instance, maybe we can look for fields that are not in some list of standard fields. Or look for other common causes. Probably needs some experimentation and looking at logs for failed queries.

Maybe also point at the documentation on https://developer.wordpress.com/docs/elasticsearch/