Fix error message for `ants.utils.transform_bbox`

# 🐜 Bug Report
[This error message](https://github.com/MetOffice/ANTS/blob/06fdecaeb3850d06cb3f2e50c6d12cae2040843a/lib/ants/utils/__init__.py#L99-L105) refers to the `bounding_box` and `target_crs`, but the variables passed in are source and target crs. The error message should be fixed.

Bonus points for updating to f-strings, and improving variable names, as per the working practices.

Open lib/ants/utils/__init__.py around lines 99-105 and inspect transform_bbox. Correct the error message so it refers to the source and target CRS variables rather than bounding_box and target_crs. If following the bonus guidance, also review the nearby formatting and variable names; done means the message accurately describes the passed arguments.

Written by the indexing model from the issue text.