Make jobCancelError and jobSnoozeError public

[jace-ys]

Hey 👋🏻 I'm not entirely sure why jobCancelError and jobSnoozeError are not exported as part of the public API, especially because jobCancelError also implements errors.Is and errors.Unwrap.

What I'm trying to achieve is to wrap my actual Work functions with an instrumented Work helper that logs/traces job errors. Right now the only way to check if the actual Work returned river.JobCancel or river.JobSnooze is to do a string prefix match, which isn't ideal.

For example:

	err = w.worker.Work(ctx, job)
	if err != nil {
		var errReason string
		switch {
		case strings.HasPrefix(err.Error(), "jobCancelError:"):
			errReason = "job cancelled"
			span.SetAttributes(attribute.Bool("job.cancelled", true))

		case strings.HasPrefix(err.Error(), "jobSnoozeError:"):
			slog.Info(ctx, "job snoozed")
			span.SetAttributes(attribute.Bool("job.snoozed", true))
			return err

		case job.Attempt == job.MaxAttempts:
			errReason = "job failed, discarded due to max attempts exceeded"
			span.SetAttributes(attribute.Bool("job.discarded", true))

		default:
			errReason = "job failed"
			span.SetAttributes(attribute.Bool("job.failed", true))
		}

		slog.Error(ctx, errReason, err)
		span.SetStatus(codes.Error, errReason)
		span.SetAttributes(attribute.String("error", err.Error()))

		return err
	}

If the error types were made public, I will be able to simplify this using errors.Is(err, &river.JobCancelError{}).

[Pascal-Delange]

Hi,
I was going to make the same ticket, independently.
This seems like a low-pain change that makes implementing a logging middleware easier.