Custom image upload

Question

I'm trying to setup upload for for attachments for my custom post form.

At first I add to my form tag enctype="multipart/form-data"

For testing purposes I have two upload forms, but in future I'm gonna implement only one form and use jQuery to append as many forms as I want.

    <fieldset class="images">
        <label for="images">Front of the Bottle</label>
        <input type="file" name="image1" id="bottle_front" size="50">
    </fieldset>
    <fieldset class="images">
        <label for="images">Back of the Bottle</label>
        <input type="file" name="image2" id="bottle_rear" size="50">
    </fieldset>

in PHP I use:

    if ($_FILES) {
        foreach ($_FILES as $file => $array) {
            $newupload = insert_attachment($file,$post_id);
            // $newupload returns the attachment id of the file that
            // was just uploaded. Do whatever you want with that now.
        }
    }

and in functions.php I have functions which should handle this form.

function insert_attachment($file_handler,$post_id,$setthumb='false') {
 // check to make sure its a successful upload
 if ($_FILES[$file_handler]['error'] !== UPLOAD_ERR_OK) __return_false();

 require_once(ABSPATH . "wp-admin" . '/includes/image.php');
 require_once(ABSPATH . "wp-admin" . '/includes/file.php');
 require_once(ABSPATH . "wp-admin" . '/includes/media.php');

 $attach_id = media_handle_upload( $file_handler, $post_id );

 if ($setthumb) update_post_meta($post_id,'_thumbnail_id',$attach_id);
 return $attach_id;
 }

everything works pefect if I just upload two images, but If I want to upload only one OR none, I get error my post_meta db:

O:8:"WP_Error":2:{s:6:"errors";a:1:{s:12:"upload_error";a:1:{i:0;s:21:"No file was uploaded.";}}s:10:"error_data";a:0:{}}

I tried inserting: if ($_FILES[$file_handler]['error'] === 4) __return_false(); in functions.php but it does not work, how to tell my script that my form is empty?

Credits: took this form from here