Patch to add Error::clone()

[Olly Betts]

I thought this feature would be useful in the bindings, but it turns out it isn't, since we always also need something based on the class name too (and not just the C++ classname as a string literal, which we can efficiently get from the error object).

But I'd implemented it before I realised that, so here's the patch in case we decide it is useful generally, or find a way to use it in the bindings.

[Olly Betts]

patch implementing Error::clone()

[Olly Betts]

We haven't found we wanted this in 9 years, and if that changes we can still dig the patch out from a closed ticket.