errors property
The form's whole error map, keyed the way client_validator.dart and a
server 422 both key it: '<name>' for a flat field, and
'<name>.<row>.<child>' for a row nested inside a repeater. error
above is this field's own entry, already looked up for convenience —
every non-repeater widget ignores errors entirely, the same as
filePicker and uploadFile are file-only. Only RepeaterFieldWidget
reads it, to put a row's own error on that row instead of folding it
into one message for the whole field.
Implementation
final Map<String, String> errors;