msgpack_rpc: Fix crash in log_server_msg

It appears that used msgpack library is not able to parse back message created 
by msgpack_rpc_from_object() if nesting level is too high, so log_server_msg now 
cares about msgpack_unpack_next() return value. Also error message from 
server_notifications_spec.lua is not readable if something is wrong (though at 
least now it does not crash when parsing deeply nested structures).

log_server_msg() in the test reports

    [msgpack-rpc] nvim -> client(1) [error]        "parse error"

0 files changed, 0 insertions, 0 deletions