code: public static <T extends Throwable> Matcher<T> withMessage(Matcher<String> matcher) {
          return IsThrowable.withMessage(matcher);
      }
docs: Throwable matcher to be used with {@link #isThrowable(Class, Matcher)}
  @see #isThrowable(Class, Matcher)
  @param matcher the message matcher
  @param <T> the throwable type
  @return the matcher