support usercode on concurrent modification exceptions

[nkuehn]

Userland code will tend to forget to do error handling for concurrent modifcation exceptions.
We should think about how to make that code as simple and minimal as possible for API user code to encourage doing it.

How? To be discussed. Could be documentation only, but on update actions one could provide helpers that analyzes the change (is it just technically colliding or does the specific change collide?) and makes it easy to "try again".

[nkuehn]

FYI: this case is separately handled from a general exception concept, because the question is whether auto-retry is possible or not.

For initial release, we will not do anything in the SDK (we will do decent exceptions nevertheless)