Improve experience for users invoking the command incorrectly

[seddonym]

From some over-the-shoulder testing:

• The uv one-liner in the README appeared to run the command in an unsupported Python version (Python 2!). Is there a way to ensure this doesn't happen?
• The user passed in a file path rather than a module name. We could clarify this in (i) the docs, (ii) the CLI help and (iii) in the error message if they pass in a module name containing slashes.