Enhancement/get all errors

[rickycheers]

Hi.

I think I'd be very nice to have a way to check if a form contains errors and a way to retrieve all of them. This would be helpful to know the state of a form and update the UI accordingly.

For example:

<?php if ($form->hasErrors()): ?>
    <span class="error">The form contains the following errors, please fix.</span>
    <ul>
    <?php foreach ($form->hasErrors() as $error): ?>
        <li><?php echo $error; ?></li>
    <?php endforeach; ?>
    </ul>
<?php endif; ?>

[coveralls]

Coverage increased (+0.05%) to 94.699% when pulling 837e799 on rickycheers:enhancement/get-all-errors into 8056cea on adamwathan:master.