PhutilErrorHandler already defines "DEPRECATED" error class, but it's not used.
I plan to use it to report deprecation warnings.

With this change, they will be shown in the Dark Console and in the error logs

Ref T15554

This change on its own won't do anything, but is safe to land first.

With D25387, pages that have deprecation warnings are now shown, and the deprecation message is available in the dark console (and in log).