Suggestion: Throw error when "maxConcurrency" is non-numeric/undefined

[Discartyptics]

If the "maxConcurrency" option in Cluster.launch is undefined somehow, Cluster never fires any workers and appears to stand still without any errors or warnings.

In my code, I had set the "maxConcurrency" option to a variable, and while refactoring code accidentally set the value to undefined - causing Cluster to never process any queue items.

Maybe Cluster could trigger a warning or error if "maxConcurrency" is set to something non-numeric?

[thomasdondorf]

Yes, this looks like an easy fix (simple if check in the constructor). Welcoming any pull requests.