args is a dictionary with the following keys:
val- The current value of this field
field_name- The name of this field
require_valid- Errors, if any.
def- This element’s definition.
This callback is called when checking the error state of a field. It allows you to intercept the error and alter it.
An error should take one of the 2 following states:
false- There is no error
string- Text description of error