Hi, Florian!

Thank you so much! I have found this repository during my research, but I didn't understand why it was needed at the time. It makes total sense now.

Now, the exception thrown might seem a little confusing. Do you think it would be viable to catch the  KeyNotFoundException  inside. TypeSerializerRegistry (or similar), and then give a better explanation? An exception saying e.g "Serializer for type ''janusgraph..xx' was not found. See documentation on custom serializers [...]" would be much more telling on where to look. 

Would you like me to add this as an issue in the Janusgraph repo?