Wp die causing 500 Internal Server Error?

Wp die causing 500 Internal Server Error?

I have function to upload images inside my settings API ,it uploads images as expected but the problem comes when user chooses to upload non-image file to stop users from uploading other files (non-image files) i am using wp_die('No image found'); an that leads to dreaded 500 Internal Server Error message displayed. Strange thing is that my wp_die wp_die('No image found'); is running fine on my local computer but not online.
function validate_setting($plugin_options){

  //echo '<pre>';  print_r($_FILES); echo '</pre>';
  $keys = array_keys($_FILES);
   $i = 0;
  foreach ($_FILES as $image) {       

if($image['size']){
    //Is it an image?
    if(preg_match('/(jpg|jpeg|png|gif)$/i',$image['type'])){
      $override = array('test_form' => false);
      $file = wp_handle_upload($image,$override);        
      $plugin_options[$keys[$i]] = $file['url'];
    }
   else{
   wp_die('No image found');//This triggers 500 Internal Server Error WHY??
       //echo:'no image found'; //Now this DON'T work locally but works ONLINE?!?
   }
}
else{
 $options = get_option('theme_options');
 $plugin_options[$keys[$i]] = $options[$keys[$i]];
}
$i++;
    }
    return $plugin_options;
    }
I tried to change wp_die() to echo 'no image found'; and this happens: now on my locall machine it echoes 'nothing' (no warning displayed), but now online it works and displays 'no image found' warning...
I know i could simply delete the else{ wp_die('No image found'); } part and display no warning, but WHY it is not working as intended?
Can someone tell me what is wrong with my code and how to display warning with wp_die('No image found') properly (without causing "500 Internal server error" )?