Add a `Stream.CancelWithErr` method.

[neilotoole]

We already have a Stream.ShutdownWithErr mechanism, which propagates a custom error.

There's also a Stream.Cancel method. However, there should be a Stream.CancelWithErr method to allow propagation of a custom error.

For example, let's say I'm reading from an http response, and writing the bytes to a stream. If there's an error reading the response, I specifically want to Cancel the stream (as opposed to calling Shutdown). But, any clients of the stream will just receive a generic ErrCanceled, which is not super helpful. I want to be able to propagate the http error back to the stream clients.

Possible implementation in PR #11.

[neilotoole]

@djherbis ^ Any thoughts on the above?

[djherbis]

I agree the with concept, I haven't had time to review the implementation though.
Work/life have been really busy with the holiday season, I'll take a look when I can though, thanks!

[neilotoole]

@djherbis Just checking in; any chance to look at the PR?