SBUS: Add a convenience function sbus_error_new
    
    Adds a convenience function that constructs a DBusError on top of a talloc
    context and as such can be used to mark an sbus request as failed without
    having to create a DBusError instance by the caller.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Stef Walter <stefw@redhat.com>
    (cherry picked from commit 87729e3a6c56383642a8d3a86b2856487f2ee064)