[BUGFIX beta] Add assertion when morph is not found in RenderBuffer.

When `_debugTemplateName` is present, the assertion is the following:

```
Assertion Failed: An error occured while setting up template bindings.  Please check "application" template for invalid markup or bindings within HTML comments.
```

When it is not present the following is asserted:

```
Assertion Failed: An error occured while setting up template bindings. Please check for invalid markup or bindings within HTML comments.
```

Closes #9365.
My thought was that the unused variable would be removed during uglification, but I updated to include the variable into the `Ember.assert`.  Should be good for re-review.