7 - FAPI: form_set_error messages on bottom of page or dialog box My ajax FAPI multi-step form is rather large, the error validation message are located by default at the top. But users can't see these messages as they are clicking submit on bottom of a large form (scroll down). 

Editing the $messages in the template has no effect on de FAPI vailidation.

Is there a way to place it in a dialog box or at the bottom?
Perhaps the Inline Form Errors module is useful to you. It provides Inline Form Errors (as the name implies). More details from the module's project page:

IFE or Inline Form Errors allows you to place form submission error inline with the form elements. Three options are provided for setting your inline error behaviour. You can configure the default behaviour or override the behaviour on a per form basis. You can add as many forms as you like.
IFE provides three behaviours for the configured forms

Leave the messages in place, this option will copy the error messages and place them inline. The original error messages set by Drupal will remain in place.
Show an alternative message, this option will replace the original messages with a generic error message such as 'Please correct all errors.'. This message can be set in the IFE configuration page. The original error messages are placed inline with the form elements
Remove all messages, this option will remove all error messages and place them inline with the form element

In all cases only the messages related to the form will be touched. All other messages will remain in tact.