If your form has only 2 fields (email and pw), you can simply reinit form.
else
{
if ($this->form->hasGlobalErrors())
$this->errorclass = 'error';
$this->form = new $class();
}
If your form has more fields (no only email and pw) and you want to preserve other field values, you can remove email and pw values and bind new form.
$values = $request->getParameter('signin');
$this->form->bind($values);
if ($this->form->isValid())
{
$this->redirect('@example');
}
else
{
if ($this->form->hasGlobalErrors())
$this->errorclass = 'error';
$values['pw'] = '';
$values['email'] = '';
$this->form = new $class();
$this->form->bind($values);
}