Enhance stacktrace logging for fatals

Enhance stacktrace logging for fatals

Split fatal error handling out of MWExceptionHandler::handleError() and
move to MWExceptionHandler::handleFatalError() which has been updated to
work as a dual purpose error handler and shutdown function. Under HHVM
it will be called as an error handler and receive a stacktrace via an
undocumented extension of the error handler callback data. Under PHP5 it
will be called as a shutdown handler and attempt to gather data via
error_get_last().

Also update the error handler installed by Hooks::run() to delegate to
MWExceptionHandler::handleError() for most errors. This will allow us to
properly handle errors raised from within hooks.

Bug: T89169
Change-Id: I0f1c66f203b91fff9069520169ecc4a3b55c43d0