Wrong RPCError: `.unknown` instead of `.cancelled`

[alex-vasenin-peggy]

Describe the bug

I noticed that sometimes if a task with grpc-swift-2 call is cancelled, it throws RPCError.unknown with the cause Swift.CancellationError().

To reproduce

I don't have a clear way to reproduce it, but it would probably be easy to catch this case in code. If not - let me know.

Expected behaviour

Cancelled tasks should return RPCError.cancelled.

[gjcairo]

Hey @alex-vasenin-peggy - thanks for filing this issue. There's definitely a problem here, but I believe it's actually two issues:

• CancellationError shouldn't actually be wrapped in an RPCError. For the codepaths in which we're doing this, we should change them to instead bubble the cancellation up.
• However, for users of this library, they may want to always deal exclusively with RPCErrors and not with CancellationErrors. So a secondary task would be to conform CancellationError to RPCErrorConvertible (with the appropriate cancelled code) so that users can simply catch it by casting to RPCErrorConvertible.