libgimpconfig: this is the appropriate fix for the previous assert.

Since the parsing failure I was experiencing was normal (not a bug), and
since I was not seeing error being passed down to deserialize(), I
assume this assert was always wrong. I hadn't realize that the GError
object was in fact populated by deserialized through g_scanner_error()
calls which were setting the GError passed at creation of the scanner
through a handler.

So there was indeed a bug in one of the functions called by our
deserialize() implementation. There was one failure case where the error
was just reported with g_warning() instead of g_scanner_error().

Now that it's fixed, I brought back the asserts (previous commits)
because the error object is actually always well populated.

libgimpconfig/gimpconfig-deserialize.c

@@ -1009,8 +1009,9 @@ gimp_config_deserialize_any (GValue     *value,
 
   if (!g_value_type_transformable (G_TYPE_STRING, prop_spec->value_type))
     {
-      g_warning ("%s: %s can not be transformed from a string",
-                 G_STRFUNC, g_type_name (prop_spec->value_type));
+      g_scanner_error (scanner,
+                       "%s can not be transformed from a string",
+                       g_type_name (prop_spec->value_type));
       return G_TOKEN_NONE;
     }
 

libgimpconfig/gimpconfig-iface.c

@@ -495,6 +495,11 @@ gimp_config_deserialize_file (GimpConfig  *config,
   gimp_scanner_unref (scanner);
 
   if (! success)
+    /* If we get this assert, it means we have a bug in one of the
+     * deserialize() implementations. Any failure case should report the
+     * error condition with g_scanner_error() which will populate the
+     * error object passed in gimp_scanner_new*().
+     */
     g_assert (error == NULL || *error != NULL);
 
   return success;