Improved error handling when GNOME Shell is not installed or not running.

1. Added check for GNOME Shell settings schema presense to avoid connector
crash.
2. Report that initialization is failed in case GNOME Shell version
property is missing on Shell bus. This is happens eg with Unity.

See-Also: https://git.gnome.org/browse/extensions-web/commit/?id=06fdd11a7be2fe29b49a1297bc281a51c28f96a0
Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=779739