Survive missing columns for default framework search fields

[pond]

The introduction of created_by as an on-by-default framework-level search filed opened up services to 500 errors when Hoodoo's finder code automatically added a created_by column search if so requested by the URI query string, but that column didn't exist (uncaught database exception typically arises, propagating up out of ActiveRecord).

Investigate ways to handle this more elegantly - e.g. detect the specific not-found-column exception case for framework related search columns and translate to a 422 instead.