Maybe EPERM or EACCESS, there are also a bunch of other errno's that may
  work but I'm not sure how if they are supported across different
  platforms and how applications would respond if they get any of these
  errors as a result of a file system syscall (open/close/stat). I
  normally try to stick to errors defined in manpages or in the single
  unix specification, but those typically don't include any of the
  following,

      ENODATA ECOMM ECONNABORTED ENOTCONN ENAVAIL ECANCELED ENOKEY
      EKEYEXPIRED EKEYREVOKED EKEYREJECTED EOWNERDEAD

While I'm a fan of following standards, it seems coda is limited to
Linux, *BSD, Mac OS X, Solaris and Windows.  As an application writer,
I'd like to be able to tell these apart

  we had authenticated path to server and server said no

  we have not authenticated path to server

separating the latter into no connectivity and no tokens seems less
interesting - in both we want to retry.