mochajs / mocha / 6790 / 7

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Pull #4133

travis-ci

Throw a descriptive error when a non-function is given to a runnable

Given a test such as:

```js
it('foobars', 4);
```

mocha used to throw the following error: `fn.call is not a function`.

While technically true, I have personally spent some minutes of my life chasing
what looked like a bug in the test itself, and not in the call to `it`. A more
descriptive error message helps bring attention to where the problem originates.

(Thanks to [ssube](https://github.com/ssube) for helping with the error wording.)

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