Code style: Fix ESLint "default-case" errors

Part of #407.

lib/compiler/index.js

@@ -66,8 +66,14 @@ let compiler = {
     });
 
     switch (options.output) {
-      case "parser": return eval(ast.code);
-      case "source": return ast.code;
+      case "parser":
+        return eval(ast.code);
+
+      case "source":
+        return ast.code;
+
+      default:
+        throw new Error("Invalid output format: " + options.output + ".");
     }
   }
 };