`Distributed.send_msg()` silently drops messages containing methods not defined on workers

Example:

using Distributed
import Distributed: worker_from_id, MsgHeader, RemoteDoMsg, send_msg
w = worker_from_id(addprocs(1)[1])

# Put `@everwhere` before this to get it to work
print_message(args...) = println(args...)

# This works
send_msg(w, MsgHeader(), RemoteDoMsg(println, ("hello!",), ()))

# This doesn't
send_msg(w, MsgHeader(), RemoteDoMsg(print_message, ("hello!",), ()))

If you put @everywhere before the definition of print_message then everything works; but if you don't, there is no error; the worker just silently ignores it. I would expect an error to tell me what went wrong. I'm not sure at what point things are going wrong; it does not appear to be a runtime error, as this occurs even when I am sending a message to call a function that is defined, but it is supposed to invoke a callback that may not be defined. Example:

using Distributed
import Distributed: worker_from_id, MsgHeader, RemoteDoMsg, send_msg
w = worker_from_id(addprocs(1)[1])

@everywhere function do_work(callback, args...)
    @info("Within do_work!")
    callback(args...)
end

# Put `@everwhere` before this to get it to work
print_message(args...) = println(args...)

# This works
send_msg(w, MsgHeader(), RemoteDoMsg(do_work, (println, "hello!",), ()))

# This doesn't
send_msg(w, MsgHeader(), RemoteDoMsg(do_work, (print_message, "hello!",), ()))

Notice how the "Within do_work" message is not emitted in the second case. My best guess is that something in the serialization code is throwing an error and that error is getting swallowed, but I haven't had time to debug this fully.

Start by running the provided Julia reproducer around send_msg, RemoteDoMsg, and the do_work callback. Trace the distributed serialization and worker message-handling path to identify where the undefined worker-side method is lost. Done means the worker no longer silently ignores the message and reports an actionable error.