I think we probably want to specify a header that implementors can use to indicate an alternate media type for errors:

```
JSONAPI-Error-Type: application/vnd.error+json
```

This kind of thing is probably how, in general, we can support extensibility when we chose an existing media type as a default that makes sense to swap out. Obviously, swapping something out will break compatibility with API browsers and clients not explicitly designed to support the alternate, but it will at least make existing clients know to ignore the data instead of trying to interpret it as the default.