fix: create storage dir on startup

[uniqueg]

• (Re)add volume definition for storage directory to prowes service in docker-compose.yaml
• Add startup routine for first start to app.py entry point, which creates the storage directory and sets the service info
• Update custom exception docstrings
• Extend error message if an unsupported workflow or version is requested
• Remove test/dummy file inadvertently pushed in f7e8879

Fixes #82