Minor ENH to espei_script.py

[Zhyrek]

Added support for single .json file to be checked:
- Previously, "espei --check-datasets datafolder/jsonfile.json" would return silently, because "datafolder/jsonfile.json" is not a folder
- Now, "espei --check-datasets datafolder/jsonfile.json" will check jsonfile.json, similarly to if you called "espei --check-datasets datafolder"

Added check that there is at least 1 .json file in the path specified
- Previously, "espei --check-datasets datafoldr" would return silently, because there are no json files at the misspelled, nonexistent "datafoldr" folder
- Now "espei --check-datasets datafoldr" will warn the user that there are no json files at the path, and maybe they have a typo

[bocklund]

Thank you!