os/exec: refactor goroutine error reporting

os/exec: refactor goroutine error reporting

Use a separate channel to report the final error of the copying
goroutines, receiving a value only when all of the copying goroutines
have completed. In a followup change (CL 401835), that will allow
waiters to select on goroutine completion alongside other events (such
as Context cancellation).

Also mildly refactor the watchCtx helper method so that its structure
better matches what will be needed to implement WaitDelay.

For #50436.

Change-Id: I54b3997fb6931d204814d8382f0a388a67b520f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/435995
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>