GetHashRange all-finished notifier should not log an error

[gavin-norman-sociomantic]

Currently, when a GetHashRange request finishes, it logs an error: https://github.com/sociomantic-tsunami/dhtproto/blob/neo/src/dhtproto/client/request/internal/GetHashRange.d#L185

This is not always an error, though, as the client might have been shut down deliberately by the user, which would cause this request to end.

A better solution might be to add a new notification type to the connection notifier.