AssertionError

In mochajs/mocha#1635, we are considering adding support for suite continuation if a hook throws an `AssertionError`.

Because **expect.js** doesn't actually throw an `AssertionError` upon failure, this feature will simply not work with **expect.js**.

Would like to start a discussion about **expect.js** throwing `AssertionError`s, much like **Chai** or **should**. I imagine:
- in a server context, use `require('assert').AssertionError`
- in a client context, the assertion library must define a global `AssertionError`. Example:

``` js
var AssertionError = function(message, actual, expected) {
this.message = message;
this.actual = actual;
this.expected = expected;
this.showDiff = true;
};
// or whatever you need to do for old browser support
AssertionError.prototype = Object.create(Error.prototype);
AssertionError.prototype.name = 'AssertionError';
AssertionError.prototype.constructor = AssertionError;
```

Would this break backwards compatibility? Any user checks of `err instanceof Error` should still work.

Review the issue's proposed server and browser AssertionError approaches and inspect expect.js's current assertion-failure behavior in both contexts. Determine whether introducing AssertionError would preserve existing Error checks and define what compatibility tests would demonstrate as done.

Written by the indexing model from the issue text.