Fell out when attempting
https://github.com/ppy/osu-server-spectator/pull/346.
Functionally, if a true non-`HubException` is produced via an invocation
of a spectator server hub method, this doesn't really do much - the
error will still log as 'unobserved' due to the default handler, it will
still show up on sentry, etc. The only difference is that it'll get
handled via the continuation installed in `FireAndForget()` rather than
the `TaskScheduler.UnobservedTaskException` event.
The only real case where this is relevant is when the server throws
`HubException`s, which will now instead bubble up to a more
human-readable form. Which is relevant to the aforementioned PR because
that one makes any hub method potentially throw a `HubException` if the
client version is too old.
Obviously this does nothing for the existing old clients.