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.

Reviewed-on: http://gerrit.openafs.org/10824
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 8beba712d95b637225f215534a759961ff4d80a9)

Change-Id: I0cf6f6e0939a1075332049361153bc8a0b0958ce
Reviewed-on: http://gerrit.openafs.org/10949
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>