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

src/applications/search/engine/PhabricatorApplicationSearchEngine.php

@@ -1142,6 +1142,12 @@ abstract class PhabricatorApplicationSearchEngine extends Phobject {
     }
 
     $constraints = $request->getValue('constraints', array());
+    if (!is_array($constraints)) {
+      throw new Exception(
+        pht(
+          'Parameter "constraints" must be a map of constraints, got "%s".',
+          phutil_describe_type($constraints)));
+    }
 
     $fields = $this->getSearchFieldsForConduit();