How do I access and create error messages - php

Question

I have a login form. If I submit it without entering anything there should be an error message over the input fields.
How can I access the error-messages from the magento controller? How should I program the messages?
<form action="<?php echo $this->getPostActionUrl() ?>" method="post">
<?php echo $this->getBlockHtml('formkey'); ?>
<div class="block-content">
<?php echo $this->__('Save your Designs');?><br> <br>
<div class="col-reg registered-account">
<div class="email-input">
<label for="mini-login" class="required"><em>*</em><?php echo $this->__('Email Address') ?></label>
<input title="<?php echo $this->__('User Name')?>" type="text" name="login[username]" id="mini-login" class="required-entry input-text" value="<?php echo $this->__('User Name')?>" onFocus="if(this.value != '') {this.value = '';}" onBlur="if (this.value == '') {this.value = '<?php echo $this->__('User Name')?>';}" />
</div>
<div class="pass-input">
<label for="mini-password" class="required"><em>*</em>test<?php echo $this->__('Password') ?></label>
<input title="<?php echo $this->__('Password')?>" type="password" name="login[password]" id="mini-password" class="input-text required-entry" value="........." onFocus="if(this.value != '') {this.value = '';}" onBlur="if (this.value == '') {this.value = '.........';}" />
</div>
<!--<div class="ft-link-p">
<a title="<?php echo $this->__('Forgot your password?')?>" href="<?php echo $this->getForgotPasswordUrl() ?>" class="f-left" target="_blank"><?php echo $this->__('Forgot your password?') ?></a>
</div>-->
<div class="actions">
<div class="submit-login">
<input title="<?php echo $this->__('Login')?>" type="submit" class="button btn-submit-login" name="submit" value="<?php echo $this->__('LOGIN') ?>" />
</div>
</div>
</div>
<div class="col-reg login-customer">
<h2><?php echo $this->__('Registration is free and easy!');?></h2>
<br>
<ul class="list-log">
<li><?php echo $this->__('Faster checkout');?></li>
<li><?php echo $this->__('save your own designs');?></li>
<li><?php echo $this->__('View and track orders and more');?></li>
</ul>
<a class="btn-reg-popup" title="<?php echo $this->__('Register')?>" href="<?php echo $this->getUrl('customer/account/create')?>"><?php echo $this->__('Create an account');?></a>
</div>
<div style="clear:both;"></div>
</div>
</form>

Answer 1

include below script in phtml file
<script type="text/javascript">
//<![CDATA[
var Form = new VarienForm('formid', true);
//]]>
</script>
to access in controller use
if (!Zend_Validate::is(trim($post['name']) , 'NotEmpty')) {
$error = true;
}
if ($error) {
throw new Exception();
}