We are having the same issue as noted here:
http://help-forums.adobe.com/content/adobeforums/en/experience-manager-forum/aem-forms.top ic.forum__zqlh-i_have_a_formbeing__dvhi-hi_i_tried_thisand._s_1_tosp.html/forum__zqlh-i_ha ve_a_formbeing.html#forum__zqlh-i_have_a_formbeing__dvhi-hi_i_tried_thisand
So trying to get errors back to the client from the server, to report to client.
We tried using post.POST.jsp with the solution posted in the above post, but we are getting already committed errors.
See "21.12.2016 15:45:28.885" in the attached log.
Switched to a form containing an adaptive form, and controlling from client lib/guideBridge via an external button I added manually.
Making progress now.
Apologies for the delay. Let me know if you face any further issues with your custom implementation.