Use "phutil_string_cast()" in TypeaheadDatasource

Summary:
Depends on D20138. Ref T13250. This improves exception behavior and gives us a standard page with a stack trace instead of a text fatal with no stack trace.

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.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13250

Differential Revision: https://secure.phabricator.com/D20140