Set AsError to TaskExceptionMissingException on FailedTask for FailedEvaluators

Non-existent Task Exceptions can occur when an Evaluator fails unexpectedly. The Task itself does not throw an Exception, so the FailedTask therefore should not contain an Exception object.

Java Task Exceptions are currently generated when AvroFailedTask.Data is set to a null byte array. Provided that the Avro schema of FailedTask does not union with null, this will never occur.

Now that C# Task Exceptions support serialization such that even when a user's Exception is not serializable we wrap a NonSerializableTaskException around it, we can assume that an empty AvroFailedTask.Data only occurs when the Evaluator fails. We should thus return null when the user calls AsError() on a FailedTask that comes from a FailedEvaluator.