Fail pipe writes when readers leave

Instead of only lazily propagating the "all readers left" error via streamcache.pipe.Close(), also eagerly propagate it via pipe.Write().

From a purely functional standpoint, it is simpler and sufficient to only propagate the error via pipe.Close(). But from an operational standpoint, it is desirable to stop writing bytes to disk as soon as we know the Close() is going to fail.

Addresses gitlab-com/gl-infra/scalability#999 (closed).