os: broaden the range of errors understood by IsNotExist on Plan 9
os: broaden the range of errors understood by IsNotExist on Plan 9

Ken's standalone file server and its derivatives, like cwfs, return
error strings different from fossil when the user opens non-existent
files.

LGTM=aram, 0intro, r
R=0intro, aram, r
CC=golang-codereviews, ken
https://codereview.appspot.com/112420045

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go