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).

gtk/gtkstyleproviderprivate.h

@@ -49,7 +49,9 @@ struct _GtkStyleProviderPrivateInterface
                                                  const GtkCssMatcher     *matcher,
                                                  GtkCssLookup            *lookup,
                                                  GtkCssChange            *out_change);
-
+  void                  (* emit_error)          (GtkStyleProviderPrivate *provider,
+                                                 GtkCssSection           *section,
+                                                 const GError            *error);
   /* signal */
   void                  (* changed)             (GtkStyleProviderPrivate *provider);
 };
@@ -69,6 +71,10 @@ void                    _gtk_style_provider_private_lookup       (GtkStyleProvid
 
 void                    _gtk_style_provider_private_changed      (GtkStyleProviderPrivate *provider);
 
+void                    _gtk_style_provider_private_emit_error   (GtkStyleProviderPrivate *provider,
+                                                                  GtkCssSection           *section,
+                                                                  GError                  *error);
+
 G_END_DECLS
 
 #endif /* __GTK_STYLE_PROVIDER_PRIVATE_H__ */