conf: handle fatal errors with GError

Don't call g_error(), which will abort the process and dump core.

This patch does not affect all the config_get_X() functions.  These
need some more refactoring.

test/run_encoder.c

@@ -73,7 +73,7 @@ int main(int argc, char **argv)
 	}
 
 	param = config_new_param(NULL, -1);
-	config_add_block_param(param, "quality", "5.0", -1);
+	config_add_block_param(param, "quality", "5.0", -1, NULL);
 
 	encoder = encoder_init(plugin, param, &error);
 	if (encoder == NULL) {