Further fix to get_an_error_message so it always returns a string.
Further fix to get_an_error_message so it always returns a string.

 - hopefully this is the last case where something other than a
   string can come out of this function;
 - may not actually make any practical difference as we just call
   str anyway which was probably what might have happened anyway.

alphagov/digitalmarketplace-search-api,alphagov/digitalmarketplace-search-api