Fail gracefully when the VAT ID validation service is not available

[andreasscherbaum]

Currently the system fails hard and doesn't let sponsors proceed, when the VAT ID validation service is not available.
Or, sponsors do not enter VAT, and then conference organizers need to add it and hopefully that happens before any invoice is issued.

A better way is to catch known problems, silently allow entering the VAT ID in such cases, and send an email that the ID must be verified and how this can be done. Less friction on both sides, and the chances that the VAT ID is invalid are small.

[andreasscherbaum]

This is becoming an issue which is preventing sponsors from wanting to sign up: https://toot.teckids.org/@nik/111811101176209360