ecm_add_wayland_client_protocol: Improve error messages
Summary: Instead of passing the data from stdin, pass the path, this way if there's an error wayland-scanner will be able to tell us the file and line where it's failing.
Test Plan: Built kwayland and kwin with it
Reviewed By: KWin, zzag
Subscribers: kde-frameworks-devel, kde-buildsystem
Differential Revision: https://phabricator.kde.org/D20105