latorante

This plugin takes care of WordPress inner comments error handling, and shows comment errors above the form, instead of using the weird grey page with single line error description. All this out-of-the-box.
It also remembers what you’ve typed in. So for example, if you fill in the comment form, and send it and forget required field, it will show the error above the form, and prefills the form with values you’ve submitted so you don’t have to retype them again (which is annoying).
By default, the plugin prints the error in this markup:
<pre><code><div id="formError" class="formError" style="color:red;">
 --error-here--
</div><div class="clear clearfix"></div>
</code></pre>
So it should be quite easy for you to style it in your css file with <code>".formError"</code> or <code>"#formError"</code>, you know the drill :). The default red colour is there just in case you won’t be able to style and since it’s error, red seems appropriate.
By the way, if you’ve customised your comment form with new fields, and you’re using the <code>preprocess_comment</code> hook, with the correct WordPress way of handeling errors, which is using <code>wp_die</code> on error encounter, this plugin will play nicely with that, and display your error messsage above the form as well.

Comment Form Inline Errors

tamlyn

As a theme developer, you want to have WP_DEBUG enabled but you’re not interested in the deprecated warnings created by other plugins. Use this plugin to disable these messages while still displaying other errors, warnings and notices.
<a href="http://outlandishideas.co.uk/blog/2012/06/hide-deprecated-errors-in-wordpress/" rel="nofollow ugc">Plugin by Outlandish Ideas</a>

Disable Deprecated Warnings

Prevents plugins from showing deprecated errors in the WordPress admin.

Comment Form Inline Errors vs Disable Deprecated Warnings