Better handling for metadata

[phoebusm]

The target of this ticket is to make data still readable even bad metadata has been written

1. Make strict_mode useful again to avoid msgpack data being pickled silenetly
2. Named error for msgpack unpack error to make user understand what has gone wrong more easily

• https://chat-man.slack.com/archives/CKD4V6N0H/p1722415251315749?thread_ts=1722246788.768089&cid=CKD4V6N0H

4. Feature flag to skip metadata deserialization

• So data with bad metadata is still readable