[FIXED JENKINS-16376] Never wait for previous builds to complete just to get a more precise status.
If and when we need to know the status of a previous build but it is still running,
simply print a warning to the (current build’s) log and proceed as if the previous build did not exist.
Merge branch 'concurrent-JENKINS-16376' of https://github.com/jglick/email-ext-plugin into jglick-concurrent-JENKINS-16376

Conflicts:
	src/test/java/hudson/plugins/emailext/ExtendedEmailPublisherTest.java