Use reflection to access protected field result from the build, see JENKINS-25406 as well.

As of https://github.com/jenkinsci/jenkins/commit/28dfd90d2d6ae99c57eb174871c46f4e07e303ba#diff-c4f9931d88bca347279b881007d71f0eL445
the field may not be set anymore directly when the build already was completed.
TODO: the summary still thinks the build was a SUCCESS.