os: handle 'has been removed' error as IsNotExist on Plan 9
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 <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh