Need to assert on identity type mismatch in FetchForWriting()

[jeremydmiller]

Had a user issue the other day. When using the FetchForWriting() functionality, the aggregate document identity type has to match the stream type in order to function. We need to make the exception be very clear about what's wrong when this happens.

[dgrozenok]

I don't remember why (maybe based on some older examples), but I had this line in the Marten configuration code for the Product aggregate:

_.Events.AddEventType(typeof(Product));

As a result the issue with the mismatch reproduced to me. When I removed that line everything started working normal. According to the description of that AddEventType method it's optional, but helps Marten in some way. How would I need to call it not to break the FetchForWriting()?