Error message placeholders aren't checked against the error's fields #169

A placeholder naming a field the error doesn't have builds:

error F {
    message = f"bad {self.nope}"
}

Validators' messages are already checked this way; an error's message isn't. The generated Display then refers to a field that doesn't exist.

Fix: check each {self.…} placeholder against the error's fields, with "did you mean".

Found on core master (a8aee68).