Custom profile fields validator

Hi.

I made a plugin for custom profile fields. How can I display errors, if name is not inserted?

add_action( 'show_user_profile', 'extra_user_profile_fields' );
add_action( 'edit_user_profile', 'extra_user_profile_fields' );

function extra_user_profile_fields() { ?>
<table class="form-table">
<tr>
<th><label for="field_name"><?php _e("Name*"); ?></label></th>
<td>
<input type="text" name="field_name" id="field_name" value="<?php
echo get_user_meta( current_user_id(), 'field_name', true);
?>" class="regular-text" /><br />
<span class="description"><?php _e("Insert name."); ?></span>
</td>
</tr>
</table>

add_action( 'personal_options_update', 'save_extra_user_profile_fields' );
add_action( 'edit_user_profile_update', 'save_extra_user_profile_fields' );

function save_extra_user_profile_fields( $user_id ) {
if ( !current_user_can( 'edit_user', $user_id ) ) { return false; }
update_usermeta( user_id(), 'field name', $_POST['field_name'] );
}