raise FileNotFoundError if blob doesn't exist

[crouchcd]

What

_download_blob_bytes should raise FileNotFoundError if the blob to download doesn't exist

Why

An error is raised: 'NoneType' object has no attribute 'download_as_string'. But not the one that was expected: FileNotFoundError: Could not find file <object_name> in <GOOGLE_ACL_DATA_BUCKET>.

How

Remarks

Checklist

Please include and complete the following checklist. You can mark an item as complete with the - [x] prefix:

• Tests - Added unit tests for new code, regression tests for bugs and updated the integration tests if required
• Formatting & Linting - black and flake8 have been used to ensure styling guidelines are met
• Type Annotations - All new code has been type annotated in the function signatures using type hints
• Docstrings - Docstrings have been provided for functions
• Documentation - README and CHANGELOG have been updated to explain major changes & new features
• Package version - Manually bumped the API package version in requirements.txt if needed