Fix the error result of the fullscreen command

Changes the error result from CMD_INVALID to CMD_FAILURE, since
CMD_INVALID indicates an unknown command or parser error and neither
occurs where CMD_INVALID was used.

sway/commands/fullscreen.c

@@ -13,14 +13,14 @@ struct cmd_results *cmd_fullscreen(int argc, char **argv) {
 		return error;
 	}
 	if (!root->outputs->length) {
-		return cmd_results_new(CMD_INVALID, "fullscreen",
+		return cmd_results_new(CMD_FAILURE, "fullscreen",
 				"Can't run this command while there's no outputs connected.");
 	}
 	struct sway_node *node = config->handler_context.node;
 	struct sway_container *container = config->handler_context.container;
 	struct sway_workspace *workspace = config->handler_context.workspace;
 	if (node->type == N_WORKSPACE && workspace->tiling->length == 0) {
-		return cmd_results_new(CMD_INVALID, "fullscreen",
+		return cmd_results_new(CMD_FAILURE, "fullscreen",
 				"Can't fullscreen an empty workspace");
 	}
 	if (node->type == N_WORKSPACE) {