Improve error message by reporting the decoded datum

[fuyufjh]

It's too painful to decode the memcomparable and value encoding manually 😇

risingwave/src/stream/src/executor/managed_state/join/join_entry_state.rs

Line 51 in 0523b41

panic!("pk {:?} should be in the cache", pk);

pk [....] should be in the cache

risingwave/src/stream/src/executor/managed_state/join/join_entry_state.rs

Line 43 in 0523b41

self.cached.try_insert(key, value).unwrap();

called Result::unwrap() on an Err value: BTree(OccupiedError…

[fuyufjh]

By the way, Join key was not printed here. Please print join key as well.