Introduce TrashedError

[noyainrain]

Introduce an own error for modifying trashed objects.

[noyainrain]

Draft:

class Trashable:
    """
    .. describe:: trashed

       A trashed object cannot be modified and a :ref:`TrashedError` is returned in this case.
    """

    def check_trashed(self) -> None:
        """Subclass API: Check that the object is not :attr:`trashed`."""

class TrashedError(ValueError):
    """Returned if it is not possible to perform an action because the object is trashed."""