Make TestFairSchedulerSystem fail with more verbose output

The TestFairSchedulerSystem test failed here: https://hudson.apache.org/hudson/job/Hadoop-Mapreduce-trunk/644/testReport/junit/org.apache.hadoop.mapred/TestFairSchedulerSystem/testFairSchedulerSystem/

with a failed assertion assertTrue(contents.contains("</svg>"));. We should make the assertion failure include the value of contents