Comment module post button creates usability issue

Currently, the "Post comment" button is not displayed if there are form errors. This creates a potential usabilty problem with 3rd party modules (like captcha that I have updated to support comments). The problem is this.

1. Drupal is configured to require previews. Captcha is configured to NOT be displayed when previews are required but only when the post button is visible.
2. User types comment and clicks "Preview comment".
3. User scans the content to verify it is correct then mis-enters the captcha code and clicks the "Post comment" button.
4. User is presented with a message that the captcha code is incorrect. However, since there are form errors, the "Post comment" button is no longer available.

In order to solve this with the captcha module, it would be necessary to have the user enter the captcha code for every submission (preview or post). This is clearly undesirable from a usability point though.

I believe it is appropriate to show the submit button even when form errors exist.

Attached is a patch to change this.