Flex-NameCdError: use instance validator

[patritzenfeld]

Validator doesn't currently accept instances without a RelationshipList, as described here: #663 (comment).

[jvoigtlaender]

In addition to calling checkNameCdErrorInstance, validation of the flex task could also encompass checking that any character mentioned in diagramIsCorrectOption indeed is the key of a "reason"/statement in the instance, and that if that statement is recorded as True in errorReasons in the instance, then no relationship is annotated with contributingToProblem = True and vice versa (the latter also with diagramIsCorrectOption = Nothing).