bosserver: Return error if key is bad

If a key is bad, have SBOZO_ListKeys return an error, rather than
silently ignoring the result from ka_KeyCheckSum.

Caught by coverity (#988413)

Reviewed-on: http://gerrit.openafs.org/9270
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit c04de52da4e89e15b211b4a19a3d9bc4d612b209)

Change-Id: I7474da0967d759fc4359140c9057ac997935dc76
Reviewed-on: http://gerrit.openafs.org/11014
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>