Use "phutil_string_cast()" in TypeaheadDatasource
Truly a great day for PHP.
(Eventually we may want to replace all (string) with phutil_string_cast(), but let's let it have some time in the wild first?)
Test Plan: Triggered the error, got a more useful exception behavior.
Reviewed By: amckinley
Maniphest Tasks: T13250
Differential Revision: https://secure.phabricator.com/D20140