Refactors setupOnerror to be a pure setup/teardown approach

[scalvert]

The first version of setupOnerror was wired up to the new hooks API setupContext/teardownContext functions, ensuring that the correct teardown occurred for each test.

The old API, however, could leak the patched Ember.onerror as the teardown would never be called.

This PR makes the utilities simple, in that it simply provides the patching capabilities. It's expected that ember-qunit will wire up the teardown in QUnit.testDone, or some equivalent. This will also be the path forward for ember-mocha, using a global afterEach.