Validators can now define a default `errorMessage` like so:
```qml
StatusValidator {
...
errorMessage: "..."
}
```
Because there's no access to runtime validation errors, `errorMessage` have to
be static. However, if applications wish to provide their own `errorMessage`
they can still override it and make it dynamic:
```qml
SomeValidator {
...
errorMessage: input.errors.someValidator ? "Whoopsie" : ""
}
```
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