Storm Django layer does not correctly capture disconnection errors when using PostgreSQL backend

The storm.django integration package allows code using the Django ORM to reuse the same database connection that Storm is using. It also tries to make sure those accesses integrate with Storm's database disconnection logic, but this is failing with the PostgreSQL backend.

Storm provides a wrapped cursor that passes all accesses through the _check_disconnect() helper to track problems:

    def _cursor(self, *args):
        cursor = super(StormDatabaseWrapperMixin, self)._cursor(*args)
        self._store._event.emit("register-transaction")
        return StormCursorWrapper(self._store, cursor)

When used with PostgreSQL, this method chains up to django.db.backends.postgresql_psycopg2.base.DatabaseWrapper._cursor, which also happens to wrap the cursor. This cursor wrapper replaces the database errors generated by the cursor with one of IntegrityError or DatabaseError from django.db.utils.

The is_disconnection_error() implementation for Storm's PostgreSQL backend does not check for Django's error objects, and also does not expect disconnections to be raised as plain DatabaseErrors (it expects them to be reported using one of the subclasses, since that is how psycopg2 behaves).

Some how, we need this is_disconnection_error() routine to trigger on these wrapped exceptions.