dir/server: change returned error back to errors.E
dir/server: change returned error back to errors.E

After second thought, we cannot use s.errPerm here. s.errPerm
will change notExist error to ErrPermissionDenied. That is not
what we want here.

Sorry for the back and forth.

Change-Id: I52597bc45043eb54139d9e05a6c8c13f0870ae0f
Reviewed-on: https://upspin-review.googlesource.com/8551
Reviewed-by: Eduardo Pinheiro <c9d6fd76b920b269b838bbb70f683c6db503d8d1@gmail.com>

upspin/upspin,upspin/upspin