Fix error on invalid concurrency

[sethp]

Else, passing in an invalid concurrency (in my case, because I wanted to see if -1 meant "unlimited") produces a confusing error message: both a limit is not a function at the point of use, and an UnhandledPromiseRejectionWarning from node itself.

An example:

const limit = pLimit(userSuppliedValue || someDefault);

...
.map(val => limit(() => doSomething()))
//               ^ TypeError: limit is not a function
...

[sethp]

A question I had is whether it would make more sense to throw the type error straightaway rather than defer it until promise resolution time?

[sindresorhus]

A question I had is whether it would make more sense to throw the type error straightaway rather than defer it until promise resolution time?

Yes. That's better. Can you update the tests?