Returns the error value in a :class:`Result`.

        Returns:
            The error value in the :class:`Result` if it is a
            :meth:`Result.Err` value.

        Raises:
            ``ValueError`` with the message provided by the success value
             if the :class:`Result` is a :meth:`Result.Ok` value.

        Examples:
            >>> try:
            ...     Ok(1).unwrap_err()
            ... except ValueError as e:
            ...     print(e)
            1
            >>> Err('Oh No').unwrap_err()
            'Oh No'

def unwrap_err(self) -> E:
        """
        Returns the error value in a :class:`Result`.

        Returns:
            The error value in the :class:`Result` if it is a
            :meth:`Result.Err` value.

        Raises:
            ``ValueError`` with the message provided by the success value
             if the :class:`Result` is a :meth:`Result.Ok` value.

        Examples:
            >>> try:
            ...     Ok(1).unwrap_err()
            ... except ValueError as e:
            ...     print(e)
            1
            >>> Err('Oh No').unwrap_err()
            'Oh No'
        """
        if self._is_ok:
            raise ValueError(self._val)
        return cast(E, self._val)