Feature: improve description of dependency requirements

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'.`

```json
"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.

No file or test is named. Start by reproducing the provided JSON Schema validation case and trace how dependency errors are formatted. Done means the message explains that “content file” is required because “raw” is present, while preserving the existing validation behavior.

Written by the indexing model from the issue text.