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.)

lib/runnable.js

@@ -338,6 +338,15 @@ Runnable.prototype.run = function(fn) {
   // for .resetTimeout()
   this.callback = done;
 
+  if (this.fn && typeof this.fn.call !== 'function') {
+    done(
+      new TypeError(
+        'A runnable must be passed a function as its second argument.'
+      )
+    );
+    return;
+  }
+
   // explicit async with `done` argument
   if (this.async) {
     this.resetTimeout();

test/unit/runnable.spec.js

@@ -670,6 +670,20 @@ describe('Runnable(title, fn)', function() {
         });
       });
     });
+
+    describe('when fn is not a function', function() {
+      it('should throw an error', function() {
+        var runnable = new Runnable('foo', 4);
+
+        runnable.run(function(err) {
+          expect(
+            err.message,
+            'to be',
+            'A runnable must be passed a function as its second argument.'
+          );
+        });
+      });
+    });
   });
 
   describe('#isFailed()', function() {