os: handle 'has been removed' error as IsNotExist on Plan 9

It fixes the TestRemoveAllRace test introduced
in CL 145860043.

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/147820044

Committer: Russ Cox <rsc@golang.org>
reflect: adjust Value.String to give correct answer for methods

Fixes issue 7859.

LGTM=r
R=adonovan, r
CC=golang-codereviews
https://codereview.appspot.com/136710043