Give more warning messages if paths can't be found

[sean-mcmanus]

I had a bogus file set for my forcedInclude (hit the same issue with compilerPath) and it didn't give any warnings. I don't think we warning for other unreachable paths in c_cpp_properties.json. Maybe we could add this as part of the settings.json work.

[bobbrow]

This was addressed with configuration squiggles work