bench.sh: fail on allocator missing or command exiting with non-zero status

CI does currently not detect when allocator libraries are missing:
https://github.com/daanx/mimalloc-bench/actions/runs/18095193097/job/51484539947?pr=243#step:4:637

Also, when benchmarks have a non-zero exit status, the test still passes: (redis and rocksdb in this log)
https://github.com/daanx/mimalloc-bench/actions/runs/18095193097/job/51484539947?pr=243#step:4:892