Create status codes for session-related errors

[fyrchik]

Currently there are 2 candidates https://github.com/nspcc-dev/neofs-node/blob/master/pkg/services/object/util/key.go#L13

[alexvanin]

I suggest to move this issue to NeoFS API repository (like #189 and #190).

[alexvanin]

Also errSessionTokenExpired may be redundant on API level. Storage node will never produce such error after nspcc-dev/neofs-node#1133. If token is expired but still available on the storage node, it can return token not found API error anyway.

[fyrchik]

I think there is still a race condition in case of token expiration, so it is possible to get expired token from storage.

[alexvanin]

I think there is still a race condition in case of token expiration, so it is possible to get expired token from storage.

It is. My point was that those errors are pretty much the same? If it is expired and if it is not found, both indicate that client has to renew session token. Maybe it can be encoded as a detail, not the separate error code. I am not completely sure about that, though.

[fyrchik]

I agree they are essentially the same, just that an error message in case of expired token can be confusing. Maybe user-facing software should just tell token was not found or has expired, this way one status is enough. Anyway both errors should be processed in an equal fashion.