browser/ember: Rethrow errors in testing mode

[Turbo87]

see emberjs/ember-qunit#304

When there is another error handler defined we still call it and rely on that one to rethrow the error in testing mode, but when there is no other handler we should make sure to rethrow the error ourselves.

Resolves #1708

/cc @rwjblue

[rwjblue]

tldr; This change looks good, thank you @Turbo87!

In general, I would expect not to have a Ember.onerror set at all during tests (but this is likely a conversation for another issue 😝 ). However, if you must define one then it definitely needs to rethrow when Ember.testing is set (or it will fail the Ember.onerror validation tests that is ran against every app by ember-qunit).

[kamilogorek]

Thanks! :)