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)

Change-Id: Ifacc9bac654d2632bd31731995795b786d485327
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>