Improve error message when --gcp_service_account_key_file is unspecified.
Improve error message when --gcp_service_account_key_file is unspecified.

PiperOrigin-RevId: 331048638

GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker