Don't abort when org.freedesktop.pulseaudio.module-group schema isn't installed

Calling g_settings_new_with_path with a schema that isn't installed raises a g_error and aborts making the KCM crash on older PulseAudio versions.
This basically copies what g_settings_new_with_path does internally but just prints a warning on console and carries on.

Differential Revision: https://phabricator.kde.org/D20915