Drupal 7 - Keep me logged in and customizing the error messages - authentication

Question

Can anybody help me in implementing Drupal 7 'Keep me logged in' feature and customize the error messages in login form.
In page--front.tpl.php i used <?php print front_login();?> for the login form and front_login() function is in template.php.
template.php functions are
<?php
function front_login() {
global $user;
if ($user->uid == 0) {
$form = drupal_get_form('front_login_form');
return theme('status_messages').render($form);
} else {
return '<div id="loginbar">' . t('Welcome back ') . ucwords($user->name) . '</div>';
}
}
function front_login_form($form, &$form_state) {
global $base_url;
$form['#id'] = 'login';
$haveAccount = '<div class="title"><span>Already have an account?</span><br>Log In Here</div>';
$forgot = '<div class="forgot">Forgot your password?</div>';
$form['#validate'] = user_login_default_validators();
$form['#submit'][] = 'front_login_form_submit';
$form['name'] = array(
'#type' => 'textfield',
'#id' => 'user_login',
'#prefix' => $haveAccount . '<div class="inputholder"><div class="icon glyphicon glyphicon-user"></div>',
'#suffix' => '</div>',
'#required' => TRUE,
'#attributes' => array('class' => array('footerinput'), 'placeholder' => array(t('Username'))),
);
$form['pass'] = array(
'#type' => 'password',
'#id' => 'pwd_login',
'#prefix' => '<div class="inputholder"><div class="icon"><img src="' . $base_url . '/' . drupal_get_path('theme', 'foodnet') . '/images/icon-2.png"></div>',
'#suffix' => "</div>",
'#required' => TRUE,
'#attributes' => array('class' => array('footerinput'), 'placeholder' => array(t('Password'))),
);
$form['keep_logged'] = array(
'#type' => 'checkbox',
'#title' => t('Keep me logged in'),
'#default_value' => 1,
'#prefix' => '<div class="row">
<div class="col-md-6">
<div class="checkbox"><label>',
'#suffix' => '</label></div>
</div>'
);
$form['actions'] = array('#type' => 'actions');
$form['actions']['submit'] = array(
'#type' => 'submit',
'#id' => 'but_login',
'#name' => 'but_login',
'#value' => t('Log In »'),
'#prefix' => '<div class="col-md-6"><div class="loginBtn">',
'#suffix' => '</div></div> </div>' . $forgot
);
$form['actions']['submit']['#attributes']['class'][] = 'btn';
$form['actions']['submit']['#attributes']['class'][] = 'btn-yellow';
$form['actions']['submit']['#attributes']['class'][] = 'btn-lg';
$form['actions']['submit']['#attributes']['class'][] = 'col-md-12';
return $form;
}
function front_login_form_submit($form, &$form_state) {
global $user;
$user = user_load($form_state['uid']);
$form_state['redirect'] = 'profile';
user_login_finalize($form_state);
if($form_state['values']['keep_logged'] ==0){
ini_set('session.cookie_lifetime', 0);
foreach ($_COOKIE as $key => $value) {
setcookie($key, $value, 0);
}
setcookie('_fnet_keepLogged', 0, time()+200000);
}else{
ini_set('session.cookie_lifetime', 2000000);
setcookie('_fnet_keepLogged', 1, time()+200000);
}
}
The login functionality works fine but I cannot customize the error messages. I have to set the errors inside the form. Please help.
When user click the 'Keep me logged in' checkbox I set '_fnet_keepLogged' cookie to 1, else to 0. I know that the default expiration of cookie in drupal is 23 days. I need to change that according to the value of '_fnet_keepLogged' cookie. Please help for this.
Thanks in advance.