[!!!][BUGFIX] DateTimeConverter returns Errors instead of throwing exceptions
As described in the Property Mapper reference manual, DateTimeConverter should
only throw exceptions when detecting developer failures. If user failures
happen, such as invalid input, an \TYPO3\FLOW3\Error\Error object should instead
be returned, as this is shown to the end-user in a friendly way, and the original
form is redisplayed.
Furthermore, DateTimeConverter must convert an empty date string ('') to NULL;
else it is not possible to define optional DateTime properties.
If you need a required DateTime property, use the @validate NotEmpty annotation
in your model.