Add easy access to error codes in forms

[Christophe Henry]

The ValidationError has a code field that can be used in views to determined what exactly failed during form validation. Unfortunately, the list of error codes in not easily accessible from form. Actually, if I want to recover the ValidationError.code`s for a form's particular field, I must write the following:

error_codes = [err.code for err in form.errors.get("field", ErrorList()).data]

I propose to add a error_codes to the Form class as follows:

@property
def errors_codes(self):
    """Return the list of error codes for each field of this form."""
    if self._errors_codes is None:
        self._errors_codes = {
            field: [error.code for error in errors.data if error.code]
            for field, errors in self.errors.items()
        }
    return self._errors_codes