fs: display cell not available on ESRCH

The cache manager pioctls abuse ESRCH to represent errors due to
unavailable cell information.  Give a more sensible error message to
the user when a pioctl returns an ESRCH error, instead of "no such
process", which is the conventional meaning of ESRCH.

The new error message is consistent with the Windows implementation
of fs.

For example, on a host with a misconfigured ThisCell and/or CellServDB.

    $ fs wscell
    fs: No such process

becomes:

    $ fs wscell
    fs: Cell name not recognized.

Change-Id: Ibdcb0957118205b9540cae07b3cafa65c51ff497
Reviewed-on: http://gerrit.openafs.org/10824
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>