Print a warning if the included config doesn't contain at least one valid section

This pull request adds support for printing a warning if the included config file doesn't contain at least one valid section.

This should make spotting issues which are related to typos a lot easier. For example, today I just spent a bunch of time debugging an issue which was related to me misspelling "program" as "progam" (yes, I do use spell checking in my text editor, but it's not so smart to recognize ":" as a delimiter).