I know it's a bit lame but you could make a constant file like this:

    # Be careful changing these strings because webpay depends on them.
    FIELD_REQUIRED = 'FIELD_REQUIRED'
    ...

This has a slight benefit where if one constant is misspelled somewhere it will trigger an import error. 

Not saying you need to do this, just an idea to make it more clear to devs not to edit the codes.