steno panics too much on consumers' mistakes

Steno currently panics when the consumer does certain things that violate our constraints. This includes things like looking up an output for a node that your node doesn't depend on. These are programmer errors, but they can be safely handled while still producing adequate debugging information.

It seems easy to have these happen in practice (by copying/pasting the wrong thing). It's really bad when sagas panic (since they will be recovered and panic again). Consumers already have to propagate operational errors that they can't directly handle (e.g., failure to deserialize an output from a previous node). Given all this, we may as well turn this particular example into a handled operational error. We should audit other uses of unwrap(), panic!(), and expect() for situations like this.

I'm not saying we shouldn't ever panic in Steno. But we may want to limit this to internal invariant violations, not incorrect usage.

Start by auditing Steno's uses of unwrap(), panic!(), and expect(), especially the output lookup case described in the issue. Separate incorrect consumer usage from internal invariant violations and trace how sagas propagate operational errors. Done means applicable consumer mistakes return handled operational errors with adequate debugging information, while intentional invariant panics remain justified.