sub add_error {
    my ($self, $fieldname, $message) = @_;

    ## if $self->error_fields->{$fieldname} doesn't exist, create it
    ## otherwise add to it.

    if (!exists($self->error_fields->{$fieldname})) {
        $self->error_fields->{$fieldname} = [ $message ];
    } else {
        push @{$self->error_fields->{$fieldname}}, $message;
    }
}