Refactor warning on bad flags.
Refactor warning on bad flags.

Makes fio_benchmark.py more readable.

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