Improve "unknown value appeared" error message

[vadz]

It's really nice that the library tries to make such an effort to produce useful and informative error messages, but bad format: unknown value appeared seems a bit underwhelming to me. I got it because I forgot quotes around a string value (e.g. I had just foo = bar instead of foo = "bar") and while I understand that there are many other possible cases in which we could fail to parse the file, it seems like this might be a relatively common mistake.

So I wonder if the error message could be changed to something like "unknown value type: expected quoted string, number, boolean etc" to be more helpful?

I could, of course, submit a trivial PR changing this if desired.