exec: better error message for windows LookPath

Fixes #1991.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/4648049

src/pkg/exec/lp_windows.go

@@ -38,7 +38,7 @@ func findExecutable(file string, exts []string) (string, os.Error) {
 			return f, nil
 		}
 	}
-	return ``, ErrNotFound
+	return ``, os.ENOENT
 }
 
 func LookPath(file string) (f string, err os.Error) {