streamline prediction destination code

[panentheos]

Summary of changes

This change is in support of an larger refactor. It removes some soft error handling in destination_for_prediction, simplifying the callers. This code has been stable for a long time, and the logs show no indication that it ever fails during normal operation. Going forward, any unexpected predictions would cause this to fail loudly, which seems like a decent approach.

[digitalcora]

Let It Crash™

Just to be clear about the failure boundaries, if this were to ever crash (and it wasn't a transient condition that resolved on process restart), what would happen with signs? Would the issue be limited only to specific signs?

[panentheos]

We've tuned the supervision layer so that individual sign crashes don't bring down the entire system, they just continuously restart (with a small delay). So the impact would be limited to signs seeing unexpected data, and they would recover automatically when the data went away.