css: Improve error reporting for variables

Emit errors for all the variables that were being expanded
if an error occurs while parsing a property value at compute time.

Include the variables that are being expanded in the error message.

gtk/gtkcssreferencevalue.c

diff --git a/gtk/gtkcssreferencevalue.c b/gtk/gtkcssreferencevalue.c
index 01ec401..cff58ed 100644
--- a/gtk/gtkcssreferencevalue.c
+++ b/gtk/gtkcssreferencevalue.c
@@ -209,15 +209,39 @@
               gpointer              user_data)
 {
   GtkStyleProvider *provider = user_data;
-  GtkCssSection *section;
+  GError *new_error = NULL;
+  GtkCssVariableValue **vars;
+  char **names;
+  gsize n_vars;
 
-  section = gtk_css_section_new (gtk_css_parser_get_file (parser),
-                                 start,
-                                 end);
+  gtk_css_parser_get_expanding_variables (parser, &vars, &names, &n_vars);
 
-  gtk_style_provider_emit_error (provider, section, (GError *) error);
+  if (n_vars > 0)
+    {
+      for (int i = 0; i < n_vars; i++)
+        {
+          if (names[i + 1])
+            g_set_error (&new_error,
+                         GTK_CSS_PARSER_ERROR, GTK_CSS_PARSER_ERROR_UNKNOWN_VALUE,
+                         "While expanding %s: %s", names[i + 1], error->message);
+          else
+            g_set_error_literal (&new_error,
+                                 GTK_CSS_PARSER_ERROR, GTK_CSS_PARSER_ERROR_UNKNOWN_VALUE,
+                                 error->message);
+          gtk_style_provider_emit_error (provider, vars[i]->section, new_error);
+          g_clear_error (&new_error);
+        }
 
-  gtk_css_section_unref (section);
+      for (int i = 0; i < n_vars; i++)
+        {
+          if (vars[i])
+            gtk_css_variable_value_unref (vars[i]);
+          g_free (names[i]);
+        }
+
+      g_free (vars);
+      g_free (names);
+    }
 }
 
 static GtkCssValue *