Improve `reduceComputed` array check assertion.

In #4078 the problem is that `reduceComputed` cannot be used against
non-extended native arrays as we cannot `addArrayObserver`.  `reduceComputed`
used to inform users of this, but no longer does so since support for non-array
dependent keys.  This commit corrects this grievous error.

Fix #4078
Merge pull request #4220 from hjdivad/reduceComputed-complain-bitterly-if-not-ember-array

Improve `reduceComputed` array check assertion.