Consider guarding against GenStage.sync_subscribe arguments about gen_stage

[fishcakez]

Hi @RobinClowers. I think this would be an issue with GenServer.whereis/1, rather than GenStage. Please feel free to do a PR adding a nice error message there. See the GenServer.start_link/3 error for an invalid :name for a suggestion for the error message if you get stuck.

[RobinClowers]

It seems like if we added when is_pid(stage) right here:

gen_stage/lib/gen_stage.ex

Line 744 in e9b13cd

def sync_subscribe(stage, opts, timeout \\ 5_000) do

I would have gotten a useful error. That would be more clear than an error from GenStage, don't you think?

[RobinClowers]

Hmm, I just saw the stage typedef, maybe this isn't as simple as an is_pid guard.

[fishcakez]

We want to allow more than just pid there though, any process that GenServer.call/3 accepts. For example :up_stage, {:global, :stage}, {:via, :stage, :left}.