Add a dedicated error subtype for halted tasks

[dcolascione]

Effection currently appears to surface a halted task as a plain Error("halted").

That makes it hard for integrations to distinguish cooperative halt/cancellation from an actual task failure. Right now the only reliable check is string-matching on "halted", which is brittle.

It would be useful to expose a dedicated subtype for halted tasks, for example HaltedError or CancelledError, so callers can handle task halt as a first-class outcome.

Related: #977, #662.

[cowboyd]

Hey @dcolascione. Thanks for submitting this! It seems reasonable since it does not change the semantics of the task at all.

I will say that one of the reasons we have been minimal up to this point is that it has almost always been a bug when we find ourselves awaiting a halted task. For that reason we've discouraged matching on halt errors in our own codebases.