Default error handler makes it impossible to test for app failures

The default error handler provided by this addon creates a trap for users by making it hard to write tests that actually fail if a page throws an exception.

The error handler will *log* the errors, but because the resulting status code is still 200, a controlling test will not be able to notice that an exception was thrown.

I think the default behavior for unhandled exceptions should be to set the status code to 500. That's what I'm doing [here](https://github.com/ember-animation/liquid-fire/blob/master/tests/dummy/app/instance-initializers/error-handler.js).