Find helper now throws when element not found and added selector context

Added selector context to `find`, `click`, and `fillIn`:

```js
find('.element');
find('.element', '.parent');
click('.element');
click('.element', '.parent');
fillIn('.element', 'new val');
fillIn('.element', '.parent', 'new val');
```

Also `find`, `click`, and `fillIn` now cause a rejection when the element is not found.  (This results in a failed assertion with 'Element not found' as reason.)