Instead of having an error member in the CSS provider's private struct,
connect a signal handler when an error is passed in. This has two
advantages:
1) It makes the code clearer as we don't have to keep track of an error
   member anywhere.
2) It causes a non-emission of the g_warning() when an error was passed
   in, because it only triggers when no signal handlers are connected.
   So we get identical behavior to GTK 3.0 where warnings where only
   emitted when no error was passed in.