LU-8797 lbuild: Fail build if zfs or spl git checkout fails

LU-8797 lbuild: Fail build if zfs or spl git checkout fails

This forces lbuild to fail if zfs or spl checkout fails, for instance
if the tag isn't present.  This turns a subtle "wrong zfs version
checked out" into a more obvious error.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I43b424a755c41cb32a7a07fa848c582bdf279e62
Reviewed-on: https://review.whamcloud.com/23563
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>