os: export errFinished as ErrProcessDone

(*Process).Signal returns an error sentinel, previously errFinished,
when (*Process).done or syscall.ESRCH. Callers would like the ability to
test for this state, so the value has been exported as ErrProcessDone.

Fixes #39444

Change-Id: I510e7647cc032af290180de5149f35ab7b09a526
Reviewed-on: https://go-review.googlesource.com/c/go/+/242998


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Tobias Klauser <tobias.klauser@gmail.com>