Revert "[BUGFIX] Changed backburner's error handler to use `dispatchE…

…rror` instead of `onError`. This is so that backburner errors can be caught by the `Test.adapter`. Fixes emberjs#14864."

This reverts commit 196442d.

packages/ember-metal/lib/error_handler.js

@@ -16,7 +16,7 @@ let getStack = error => {
 let onerror;
 export const onErrorTarget = {
   get onerror() {
-    return dispatchOverride || onerror;
+    return onerror;
   }
 };