Handle concurrency of 0

[jgornick]

When specifying a concurrency of 0, unexpected results happen. If passing 0, I'd expect an error to be thrown, or the function is not throttled.

[ForbesLindesay]

Happy to accept a pull request for throwing an error.

[cloverzrg]

i hope no throttled when passing 0

[ForbesLindesay]

I'd rather throw an error.