Added _drupalgap_form_submit_response_errors().
Changed drupalgap_entity_form_submit() to use _drupalgap_form_submit_response_errors().
Changed user_register_form() to add a 'confirm e-mail' field if e-mail verification is not required on the site. Added error handling to user_register_form_submit() with _drupalgap_form_submit_response_errors().