More updates

A few more tweaks and improvements to the handling of files in subfolders.

- Files checked with the check_files method can also be treated as subfolders, and files within them will be recursively added, same as if a folder name is passed on the command line.

- Files beginning with "test_" are properly handled when found in subfolders.

- Filenames are displayed with their full absolute path in the error output.

EDIT:  Also added:

- Token errors (i.e. caused by running on a non-python file) are handled gracefully