memory: Use i18n messages for memory-related errors
msgpack-rpc: Remove the `msgpack_rpc_unpack` function

The `msgpack_rpc_unpack` function was created to work around a deficiency in the
msgpack unpack API, which did not let the caller know if parsing failed due to
needing more data or to invalid input. The deficiency does not exist in the
latest version of `msgpack_unpacker_next`, so it can safely be removed.