Warn if no files are found

[twavv]

This is relatively minor, so I'm going to skip the template. Hope that's okay.

I opened this issue on TailwindCSS: tailwindlabs/tailwindcss#3770 (comment)

tl;dr: PurgeCSS should warn if globs don't match any files. This will help catch issues like mine where directories are misspelled (in my case, I forgot to add the ./src/ prefix).