[JENKINS-16845] Properly report the IllegalStateException, rather than...

[JENKINS-16845] Properly report the IllegalStateException, rather than throwing an NPE producing the exception message.
java.lang.NullPointerException
	at hudson.model.Run.getFullDisplayName(Run.java:711)
	at hudson.model.Run.toString(Run.java:706)
	at java.lang.String.valueOf(Unknown Source)
	at java.lang.StringBuilder.append(Unknown Source)
	at hudson.model.AbstractBuild.getPreviousBuild(AbstractBuild.java:218)