Match implementation with documentation on error message

[maischon]

/**
 * Maps an assertion on a [Throwable] to an assertion on its
 * [Throwable.message].
 * This mapping also asserts that the message is not `null`.
 *
 * @author [Xavier Hanin](https://github.com/xhanin)
 */
val <T : Throwable> Builder<T>.message: Builder<String?>
  get() = get("message") { message }

The idea described in doc comment is good, but the implementation still returns String? (nullable)