Incorrect diagnostic for unrecognised indexes

When a query like materialType="1a54b431-2e4f-452d-9cae-9cee66c9a892" sortby title is submitted to mod-inventory, the query is rejected with a diagnostic:

CQL State Error for 'materialType="1a54b431-2e4f-452d-9cae-9cee66c9a892" sortby title': org.z3950.zing.cql.cql2pgjson.QueryValidationException: cql.serverChoice requested, but no serverChoiceIndexes defined.

This message is incorrect. Instead, the specific invalid index should be named in a message along the lines of "invalid index 'materialType'".

In CQL, there are two ways to use the cql.serverChoice index: explicitly, of course (as in a query like cql.serverChoice=water) and implicitly by including a query term with no index specified (as in a query like water). Since the queries in this example contain no terms that are not directly associated with an index, cql.serverChoice should not come into play at all.

(This issue came up in connection with, but is distinct from, UIIT-50.)