Level "warning" versus level "fatal" on elements in syntaxes not part of the EN16931

[AndreasPvd]

With the introduction of mandatory e-invoicing in Germany companies start mixing CIUS and Extensions. The resoning is that the validation artefacts just throw a "warning" on extension elements.

While validation artefacts for extensions definitively allow the use of additional elements the standard-en16931-artefacts should not.

I propose to change the severity level for UBL and CII syntax checks for elements not being part of EN16931 from warning to fatal.

[phax]

From my point of view, the sage of warnings on data elements is fine.
The crucial parts are the "rule" checks - they must always be fatal. Adding additional rules that do not break any of the existing stuff is bad but not too bad.

[phax]

This is a duplicate of #415

[AndreasPvd]

But exactly that is the case. As a result German legislation makes a bilateral agreement mandatory once additional elements are used. Some existing extensions break EN rules but are still conformant to EN rules for extensions. Having in mind the many discussions about mandatory handling of additional information that is not part of the standard it would be better to have clear validation artefacts.