[FIXED JENKINS-16376] Do not block on build step monitor

[JENKINS-16376](https://issues.jenkins-ci.org/browse/JENKINS-16376) simplest suggestion: print a message to build log when the previous build is still building, then pretend that we are the first build.

(Could easily be modified to go back to the prior completed build, if any, but this seems potentially more confusing, and probably not useful.)