Error propagation in C

[Cryoris]

(How) do we want to propagate internal Rust errors to C? Our current best guess is to have structs represent a result, which contains an error code and the result. Something like

struct ObservableResult {
    int error;  // 0 if no error, otherwise an error enum
    SparseObservable *result;
}

currently the code will just panic.