perftune.py: introduce a PerfTunerBase.CPUMaskIsZeroException
perftune.py: introduce a PerfTunerBase.CPUMaskIsZeroException

This is a special exception type that is thrown when configuration
parameters result in a zero compute CPU set.

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@scylladb.com>

dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar