The search settings form was assuming that the search_backend_id field

would be available in clean, but the value is only present if the

backend passes validation. We now check for its existence instead of

assuming that it does exist and carry on with validation accordingly.

Ran unit tests.
Selecting "elasticsearch" on the search settings form without it

  installed no longer results in a 500.
Enabling Elasticsearch works.
Enabling Whoosh works.