razor view exception bug - status code not set to 500

started with a discussion at https://groups.google.com/forum/?fromgroups=#!topic/nancy-web-framework/hGvydkNnQ9w

If I add a view with the following code. the error handler will not be executed cause the status code is set to 200ok instead of 500.

```html
@using System
@{
    throw new NotImplementedException();
}
```

Will also need to add `context.Items["ERROR_TRACE"]` and `context.Items["ERROR_EXCEPTION"]`.  Most likely even other view engines have this bug.

This PR does not contain the fix, but only a test showing how it fails.