Small fixes in fio_benchmark.py
Small fixes in fio_benchmark.py

Change a sys.exit(1) call to an explicit exception and eliminate an
unneeded function parameter. Thanks to review comments.

meteorfox/PerfKitBenchmarker,kivio/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,kivio/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker,syed/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,syed/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker