Add suggestion to use a partial to "Undefined variable" error message

So I'm fairly new to Sass, and I didn't know about partials. I was trying to import a Sass file, and suddenly my variables weren't working. I started googling (which was successful in the end) but a pointer such as the one I'm suggesting in the commit would've helped quite a bit, since I would've had a keyword to google for.

I know that this is a definite case of RTFM, but I imagine that this might not be an uncommon issue for peope new to Sass, since it is not super-intuitive, imho.

Ideally, the error message mentioning the partial should only be displayed when actually inside an import, but I didn't find a way to determine if that is the case. If someone would be so kind as to give me a hint on how I can detect that, I'd be glad to change the PR :smile: