Improve scroll to mandatory field when form is submitted with errors

Currently the scrolling is sending the mandatory errored field really at the top of the browser. 1. it's ugly. 2. it gives issues with non-fixed top menus.

This commit fixes this by scrolling the mandatory errored field to the middle of the screen. It works well with the default install (haven't tried other special custom designs), for both new inserting design and legacy one.