Example: https://pantheon.corp.google.com/cloud-build/builds;region=global/c61222c7-576d-467c-9c78-38100c413b09?e=RetailSupportToolLaunch::RetailSupportToolEnabled,RetailDataQualityGaLaunch::RetailDataQualityGaEnabled&mods=allow_workbench_image_override&project=cloud-sdk-librarian-prod

In the above example, we attempt to generate all APIs for google-cloud-python. There is only a single library configured and generation fails.

The CLI should exit with non-zero code and should not open an empty PR.