[BUGFIX beta] Fix error with $.focus() on Firefox.

Firefox does not support calling `.focus()` if the window itself does
not have focus.

Resolves #3521.
[BUGFIX beta] Make Component.sendAction behave the same as {{action}} helper.
Use Ember.Error consistently.
[BUGFIX beta] Fix evaluateUnboundHelper properties
[BUGFIX beta] Throw a sensible exception from SubArray.removeItem when not found.

- Throw a descriptive exception from SubArray.removeItem when there are
  no operations in the SubArray.
- Test that the correct exception is thrown from SubArray.removeItem.