Throwing exception when a non-function callback is passed to elm.ports[port].subscribe

[emilianobovetti]

Currently calling elm.ports[port].subscribe(undefined) correctly adds undefined to subscribers list. This will produce the following exception when subscribers get called:

TypeError: currentSubs[i] is not a function

I think this behaviour should be changed because that exception message can be confusing, the stacktrace contains only Elm code, but most important the error could be raised before pushing the callback in subscribers list.

Following the same logic as send, that throws a runtime exception immediately when it receives invalid data, the subscribe with this PR will raise this error:

Error: Trying to subscribe an invalid callback on port `<port name>`

Why I think typeof callback !== 'function' is safe:
According to ECMAScript language specification the function call expression currentSubs[i](value) will throw an error if IsCallable is false, which returns true if its argument has the [[Call]] internal method. typeof will return "function" if an object implements [[Call]].

If I'm not wrong, it's impossible that typeof doesn't return 'function' and currentSubs[i](value) doesn't raise a TypeError, so if this is true I don't see any downside in throwing an error immediately!

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

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