Improve validation results error description

Currently, a validation results error produces output in a hard-to-understand dictionary: https://github.com/E3SM-Project/zppy/blob/main/zppy/__main__.py#L171

Suggestions for improving the error description:
- Pretty print the dictionary
- Print only the keys/values with `False` in the validation dictionary
- Print a link to the tutorial, which has examples of how values should look (but not for all parameters...)
- See if there's any way for the `Validator` to give more information on why it failed