Before this patch mp_error API could only encode and decode
hardcoded fields from the C++ classes inheriting struct error.

The fields are gone from the classes in a previous patch - moved
into error_payload. Now to be able to support arbitrary fields in
the payload the MessagePack encoding/decoding must use its content
instead of hardcoded fields depending on error type.

Part of #5568
Part of #4610
Part of #4907