The return value can be one of three types:
- an arbitrary object, or a simple type (which has been created while mapping).
This is the normal case.
- NULL, indicating that this object should *not* be mapped (i.e. a "File Upload" Converter could return NULL if no file has been uploaded, and a silent failure should occur.
- An instance of \Neos\Error\Messages\Error -- This will be a user-visible error message later on.
Furthermore, it should throw an Exception if an unexpected failure (like a security error) occurred or a configuration issue happened.