GraceQueryField/SimpleSearchForm builds db query.

GraceQueryField used to return a Q() instance for cleaned 'query'
field.  Unfortunately, bad queries were not always identified until
the Q() instance was used to do a query.  If the filter() is done
while cleaning the data, this error case can be identified earlier
and in what is likely a more appropriate place.  Luckily, Django
querysets are lazy, so only the SQL query is build -- nothing hits
the database itself until the data is required.