CiphertextType.deserialize should throw an error if it encounters an invalid type

[mattsb42-aws]

Currently, if CiphertextType.deserialize is called with a byte containing anything other than a valid ciphertext type ID, it will return null. This seems like the wrong behavior to me.

https://github.com/aws/aws-encryption-sdk-java/blob/master/src/main/java/com/amazonaws/encryptionsdk/model/CiphertextType.java#L71-L75