[Frozen Architecture] Define behaviour of closing Realms inside Flows

[cmelchior]

Currently, closing Realms inside Flows has undefined behavior. We need to define and implement the semantics we want for this.

TODO

• Define the semantics we want. We probably need to play around with some prototypes. Some possibilities: A) Throw like we do for MutableRealm B) Allow it, and assume people know what they are doing
• Tests

[sync-by-unito]

➤ Christan Melchior commented:

With frozen architecture, there is always a race condition here. So instead we should rely on Core catching this and map its errors in a way that makes sense to users.