Changeset [cde179786cfe3647fbde646b3a5a2c26d9a5ef09] by Klaas Pieter Annema

fail more gracefully when attempting to remove the same observer twice

Before this commit we would fail gracefully for single keys. For key paths an application would crash because we would try to get the forwarder property of an undefined object. This commit makes sure we don't attempt any removal and logs a warning (just like Cocoa) about the attempt to remove an unregistered observer.
https://github.com/280north/cappuccino/commit/cde179786cfe3647fbde6...