msgpack-rpc: Return from msgpack_rpc_validate on error

- When validating a msgpack msg we need to return on the first error
  otherwise we can SEGFAULT with invalid checks
msgpack-rpc: Fix typo in validation message
msgpack-rpc: Don't try to write into a closed channel
msgpack-rpc: Close channel on invalid msgpack RPC

- When an invalid msgpack RPC msg is received from a channel
  we now close that channel all calls on that channel fail with
  an error message.
Merge PR #1399 'Better handling for invalid msgpack-rpc'