Adds improved error/warning mesages
doc-md will now fail with a useful warning if there's a directory without a properties.yml file or with an invalid properties.yml file.
It will now warn the user if a properties.yml file doesn't define a toc
It will fail if the toc contains an entry with no file property or a non-directory entry with no name
It will warn the user if any of the parsed markdown contains headers (which could conflict visually with the headers created by doc-md).