Form validation error not outputted in PyroCMS - codeigniter

Question

Using PyroCMS 1.3.1 I've built a model that's pretty much a copy-paste version of the included contact model, but with a few tweaks. When all fields are inputted correctly everything works as expected. If a field is left out or incorrectly filled in, the form does not submit - just as expected.
However, I can't seem to get the form validation message to be outputted and this is driving me crazy. I'm sure I've just missed something very basic so if anyone could point it out I'd be grateful.
View file (form.php) contains this
<?php if (validation_errors()): ?>
<div class="error-box">
<?php echo validation_errors(); ?>
</div>
<?php elseif (isset($messages['error'])): ?>
<div class="error-box">
<p><?php echo $messages['error']; ?></p>
</div>
<?php endif; ?>
Controller (plugin.php) looks like this
class Plugin_mycustommodule extends Plugin {
private $rules = array(
array(
'field' => 'firstname',
'label' => 'lang:mycustommodule_firstname_label',
'rules' => 'required|trim|max_length[80]'
),
/* ... snip ... */
array(
'field' => 'license',
'label' => 'lang:mycustommodule_license_label',
'rules' => 'required'
)
);
public function __construct()
{
$this->lang->load('mycustommodule');
}
function form()
{
$this->load->library('form_validation');
$this->load->helper('form');
$this->form_validation->set_rules($this->rules);
// If the user has provided valid information
if ($this->form_validation->run())
{
/* ... Custom processing here ... */
// The try to send the email
if ($this->_send_email())
{
$message = $this->attribute('confirmation', lang('mycustommodule_sent_text'));
// Store this session to limit useage
$this->session->set_flashdata('success', $message);
redirect(current_url());
}
else
{
$message = $this->attribute('error', lang('mycustommodule_error_message'));
$data['messages']['error'] = $message;
}
}
// Set the values for the form inputs
foreach ($this->rules as $rule)
{
$form_values->{$rule['field']} = set_value($rule['field']);
}
$data['form_values'] = $form_values;
return $this->module_view('mycustommodule', 'form', $data, TRUE);
}

Answer 1

So it turns out that while I was working on customizing the CodeIgniters language files I must have messed up the upload of form_validation_lang.php because all entries was empty i.e. $lang['required'] = '';
So basically the validator looked for the error message, found an empty string, which was trimmed from being outputted. As suspected something silly, just not in the place I expected.
Let's hope this post will save someone else the trouble.