When an invalid "constraints" parameter is provided to a "*.search" method, raise a more tailored error

Summary:
See https://discourse.phabricator-community.org/t/constraints-parameter-type-handling-in-maniphest-search/2636.

(The caller provided constraints={...} via cURL, but you can't mix JSON and HTTP parameters like that.)

Test Plan: Called curl 'http://local.phacility.com/api/maniphest.search?api.token=...&constraints=X' (with a valid API token), got a more sensible error.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20429