Merge pull request #65 from jglick/messaging-JENKINS-45245

[JENKINS-45245] Improve messaging

src/main/java/org/jvnet/hudson/test/WarExploder.java

@@ -84,7 +84,7 @@ private static File explode() throws Exception {
         URL winstone = WarExploder.class.getResource("/winstone.jar");
         if(winstone==null)
             // impossible, since the test harness pulls in jenkins.war
-            throw new AssertionError("jenkins.war is not in the classpath. If you are running this from the core workspace, run 'mvn install' to create the war image in war/target/jenkins");
+            throw new AssertionError("jenkins.war is not in the classpath. If you are running this from the core workspace, run 'mvn install' to create the war image in war/target/jenkins. If running from a plugin, try using `mvn clean test` (cf. JENKINS-45245).");
         File war = Which.jarFile(Class.forName("executable.Executable"));
 
         // TODO this assumes that the CWD of the Maven process is the plugin ${basedir}, which may not be the case