Improve client side script

- Moved form submit handler to the global wplf object
  - You weren't able to easily remove anonymous event listeners, and if you removed it the hard way, good luck adding it back. 
  - Now you can remove said listener like this: `form.removeEventListener('submit', window.wplf.submitHandler)`, adding works the same way. 
- Print a warning message to the console only if there's no errorCallback
- Fixed indent
- improved readability (and ruined diffs as a side effect)

Author: k1sul1