Add a shortcut for getting error message of validation

This function is a shortcut to getting the error message.
This is used in the following cases.

```php
<?php

class Controller_Test extends Controller_Rest
{

	public function get_list()
	{
		$val = Validation::forge('test');
		$val->add_field('from', 'from', 'required|valid_date');
		$val->add_field('to', 'to', 'required|valid_date');

		if ( ! $val->run())
		{
			$this->response(array(
				'result' => 400,
				'error_messages' => $val->error_message()
			));
		}
		else
		{
			// validated..
		}
	}
}
```