Provide better user feedback when a Domain definition is invalid

[pcdavid]

We currently have some validation applying to domain definitions (see org.eclipse.sirius.components.domain.emf.DomainValidator), but when converting a domain definition into an actual EMF EPackage we are much more strict than what these validation rules check for.
Indeed, in DomainConverter we only consider domains which, once converted, do not have any error:

// Only return valid and safe EPackages
return ePackages.stream().filter(ePackage -> Diagnostician.INSTANCE.validate(ePackage).getSeverity() < Diagnostic.ERROR);

In practice this means even with no visible validation error, a domain definition can fail to be converted and thus not deployed/available, with zero feedback to the studio maker about what is wrong and needs to be fixed.