Implement warnings for missing playpen files

[projektir]

mdBook currently doesn't seem to say anything if I try to include a file that doesn't exist in a playpen. It would be nice to have a warning for that. This is a nice to have for #308.

There's a warning here, but I'm not entirely sure where it's meant to show up.

I think a good place for this would be mdBook test, unless we want this to be a build failure? mdbook test seems like a good place for various checks on the final document, even if it would involve extra work to find playpen links all over again. Could this be part of the linkchecker?

[azerupi]

I think logging somehow got disabled at some point, that's not good.

I think a good place for this would be mdBook test

I think this is definitely something that should spawn at least a warning at build or even an error. Actually we could have a new config flag deny-warnings or something similar that would let you choose between warnings or errors.

I wouldn't put it in mdbook test alone because I'm not sure it is used that much, but it could be a check both in mdbook build and mdbook test