Handle missing search parameters header

Previously, a missing header would have caused an exception.  Slimmer
would have then handled the exception, so the page would have worked,
but reports would have been sent to errbit.

I believe this will fix the problem, but I can't see an easy way to add a test for this, because slimmer catches any exceptions in processors.  Advice welcome (though less welcome if it says "add a whole new unit testing framework to slimmer").