butc: Catch failures to start keep alive thread

If we can't start the keep alive thread, in either the LWP or
pthread versions of a code, report an error and exit instead of
carrying on regardless.

Caught by clang-analyzer

Reviewed-on: http://gerrit.openafs.org/9245
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit e4862e284832420e9dbf5a6f437f6f7c8801f7dd)

Change-Id: I55453557b592bb1814877d291b1604907ab2e8d6
Reviewed-on: http://gerrit.openafs.org/9535
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>