Field errors

This enables Luca to display server-returned errors on form fields. Please tell me what your thought is about this approach.

We've seen that Field already have `statuses`:
```coffee
statuses: [
  "warning"
  "error"
  "success"
]
```
That `statuses` looks related but we were not really sure, because a child could remove its support for errors, and that is not what we want, or is it?