#1993 - Remove "failed" query string from upgrader redirect if there are no errors.

- Changed redirect if it finished without failures.
- No change to Upgrader_Controller::index(), since its behavior with an empty vs. undefined failed query is identical.

... and saved people like me from being freaked out when they upgrade their 35,000+ image production site to the newest development version only to see "failed" in the address bar :-)