Throw an exception if `ready` is greater than `total`

[bradberger]

One somewhat helpful feature of WaitGroup in go is that it throws a panic if the counter is negative. Would like to suggest throwing an exception under the same circumstances in this library.

Would be happy to submit a PR, it should only need a line or two.

[dtjm]

Hi @bradberger, yes I'd be happy to merge that PR 😄