code: public @NotNull ThrowableAssert hasMessage(@Nullable String message) {
      isNotNull();
      failIfNotEqual(customErrorMessage(), rawDescription(), actual.getMessage(), message);
      return this;
    }
docs: Verifies that the message of the actual {@code Throwable} is equal to the given one.
  
  @param message the expected message.
  @return this assertion error.
  @throws AssertionError if the actual {@code Throwable} is {@code null}.
  @throws AssertionError if the message of the actual {@code Throwable} is not equal to the given one.