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://golang.org/cl/112420045

src/pkg/os/error_plan9.go

@@ -25,7 +25,7 @@ func isNotExist(err error) bool {
 	case *LinkError:
 		err = pe.Err
 	}
-	return contains(err.Error(), "does not exist")
+	return contains(err.Error(), "does not exist") || contains(err.Error(), "not found")
 }
 
 func isPermission(err error) bool {