Fix explosion in Ember.SubArray's _findOperation when no operations are present in the SubArray.

While working on #3518 I came across a problem where if you call `removeAt` on a sub array with no operations, then it explodes:

```javascript
subArray = new Em.SubArray();
subArray.removeItem(2);
# TypeError: undefined is not a function
```

This is because SubArray's `removeItem` function calls `_findOperation` without providing the second, unhappy-path callback.  Now, I'm not sure if this should blow up (in a more sensible way) or simply return `-1`.  Perhaps @hjdivad can shed some light?

In the mean time I have patched `_findOperation` not to call `notFoundCallback` when it is undefined and written a test to verify that -1 is returned. Would this be a good use for promises?