Feature: improve description of dependency requirements

[plocket]

One of my error messages is because my attachment["raw"] requires attachment["content file"]. The most specific error I get is /attachment/0 must have required property 'content file'.

    "attachment": {
      "dependencies": {
        "raw": {"required": ["content file"]},
      }
    }

It'd be great to know that content file is required because raw is present. Not sure if it's possible, but I can dream.