Hoist validation errors from hidden fields to the top of the form

Hoist validation errors from hidden fields to the top of the form

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