Error handling of Session.execute in modelled! macro

[jvmncs]

During #520, we found it would be good to catch compilation errors for kernels in the modelled! macro (discussion). This was out of scope of #453, but should be addressed in follow up work.

[jvmncs]

Note that this issue would require introducing error handling to all modeled ops, for example PlacementMean::mean (as well as other modeled traits) would now return Result<O> instead of O.

[eric-capeprivacy]

what about other modelled traits such as

pub trait PlacementDeriveSeed<S: Session, KeyT, SeedT> {
    fn derive_seed(&self, sess: &S, sync_key: SyncKey, key: &KeyT) -> SeedT;
}

[eric-capeprivacy]

will probably jump on this again now that Morten introduced a new Macro