CSS provider: Add a way to emit errors

CSS provider: Add a way to emit errors

Currently, GtkCssProvider can emit ::parsing-error only during
the actual parsing, although the documentation hints that it might
happen at other times.

This commit adds a emit_error method to the GtkStyleProviderPrivate
interface that will let us emit errors from the compute() implementations
as well, which can be useful (e.g. if an image fails to load).