Fix GH-8750: Can not create VT_ERROR variant type

We add support for creating `VT_ERROR` variants via `__construct()`,
and allow casting to int via `variant_cast()` and `variant_set_type

Fix GH-8750: Can not create VT_ERROR variant type

We add support for creating `VT_ERROR` variants via `__construct()`,
and allow casting to int via `variant_cast()` and `variant_set_type()`.
We do not, however, allow type conversion by other means, to avoid
otherwise easily introduced type confusion. VB(A) also only allows
explicit type conversion.

We also introduce `DISP_E_PARAMNOTFOUND` which might be the most
important `scode` for this purpose, since this allows to skip optional
parameters in method calls.

Closes GH-8886.

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