Include better error handling

Currently, error handling is informal where it exists.

In find_ridge, errors should be raised for unsupported values of fittype, kde_bwmode, and kde_error keywords.

man_dat.py will also require error handling to be included, especially for missing or weird data

Start by reading the find_ridge entry point and man_dat.py to map the existing informal error handling. Define the supported values for fittype, kde_bwmode, and kde_error, then address missing or unusual data in man_dat.py; done means unsupported keyword values and problematic data produce explicit errors.

Written by the indexing model from the issue text.