HTMLFormField subclasses are supposed to handle error display but
some (like hidden fields) have no means of doing this. Add
a HTMLFormField::canDisplayErrors() method which can be overridden
to return false, in which case HTMLForm will take care of the
error display.
Also adds a 'rawmessage' message which can be used to wrap
arbitrary text. This can be passed to methods which expect a message
specifier array but do not allow a message object (so the RawMessage
class cannot be used), such as HTMLFormField::trySubmit().
Bug: T112635
Change-Id: I5d73536805774ff2ee0ec64b5442650c4888dc84
HTMLHiddenField is unable to output a non-default value, which makes it
unsuitable for actually persisting changes across multiple form
submissions. To preserve compatibility with forms that aren't expecting
persistence, fixing this requires a new parameter in the field
definition.
Also, due to the unusual way that hidden fields are added to the HTML
output, they are not being correctly handled by HTMLFormFieldCloner.
Special-case that.
Change-Id: I1fde7372401299b4432d28ac61982d47d5f3bbea
