butc: Catch failures to start keep alive thread

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

Change-Id: I0a25a236aadeedf10d3c0c129d716b1397c9a4e3
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>