feat(parser,execution-engine): allow :error: in fail (#696)

The `(fail :error:)` was previously rejected by parser.  Now it is
executed, but is of limited use.

crates/beautifier/src/tests/beautifier.rs

@@ -381,3 +381,10 @@ fn deeply_nested() {
     let expected = include_str!("deeply_nested_expected.txt");
     assert_eq!(output, expected);
 }
+
+#[test]
+fn fail_error() {
+    let script = r#"(fail :error:)"#;
+    let output = beautify_to_string(script).unwrap();
+    assert_eq!(output, "fail :error:\n");
+}