Make error kind public

[hn-sl]

Sometimes application needs to take different action on different kind of errors but there is no easy way to differentiate errors when cause is None (#583).

[sfackler]

Which kinds of error in particular do you want to handle differently?

[hn-sl]

Which kinds of error in particular do you want to handle differently?

I think in some cases at least Kind::Closed should be handled differently. For example, if query execution is failed by Kind::Close error the application might want to connect to database few seconds later and try the query again. This behaviour is reasonable for many applications because connection lost is not critical error. But currently there is no easy way to check if the failure was caused by Kind::Closed.

Also this can be used to detect connection lost. I know that task from Connection object is terminated as soon as connection is lost, but it would be helpful if connection lost could be detected by error from client method.

[hn-sl]

What do you think about this issue?

[sfackler]

I'd be happy to take a PR adding an is_closed() method to Error, but I do not want to expose its internal structure. A direct method will be easier to work with for your use case anyway.

[hn-sl]

Okay I just removed previous commit, synced with latest branch and add is_closed() method. Could you check this update?

[sfackler]

Thanks!