Explicitly fail if phutil_build_http_querystring() is passed a map of nonscalars
See D20114. Somewhat (un-?) surprisingly, http_build_query() accepts an arbitrarily deeply-nested mess of arrays and encodes them in a PHP-specific way.
We generally don't want this to happen, so fail explicitly when it does.
Test Plan: Published a story on master (without D20114) with feed.http-hooks enabled, got a more helpful/explicit error.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20116