Better error handling

src/prelude/ensure.ts

@@ -3,7 +3,7 @@
  */
 export function ensure<T>(x: T): NonNullable<T> {
 	if (x == null) {
-		throw 'ぬるぽ';
+		throw new Error('ぬるぽ');
 	} else {
 		return x!;
 	}