Improve shouldRejected reason

Currently:

```js
shouldRejected(Promise.resolve("VALUE")).cath(function(error){});
// => "Expected promise to be rejected but it was fulfilled"
```

We don't know what's `promise` fulfilled **value**.

This pullrequst improve the error message.

Proposal:

```js
shouldRejected(Promise.resolve("VALUE")).cath(function(error){});
// => "Expected promise to be rejected but it was fulfilled: VALUE"
```