Add tests and documentation for JSON error and warning output.

This PR addresses a couple of @nex3's [requests](https://github.com/nex3/sass/issues/593#issuecomment-11631340) on nex3/sass#593. 

I will submit a different pull request to add metadata to SyntaxErrors, but I'm less sure of how that work will end up. 

I'm just adding tests and docs here. There is a persistent test failure `test_extend_sourcemap_scss` unrelated to this change. 

Hope you approve :)