Attempting to be more robust against problems initializing this class.

https://jenkins.ci.cloudbees.com/job/core/job/jenkins_main_trunk/896/testReport/junit/hudson.tools/ToolLocationNodePropertyTest/testFormRoundTrip/
java.lang.NoClassDefFoundError: Could not initialize class hudson.model.ComputerSet
	at jenkins.model.Jenkins.getComputer(Jenkins.java:1107)