error message improvement.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@2302 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/maven/MavenBuild.java

@@ -284,7 +284,7 @@ public class MavenBuild extends AbstractBuild<MavenModule,MavenBuild> {
                 }
             });
             if(classworlds==null || classworlds.length==0)
-                throw new IOException("No classworlds*.jar found in "+bootDir);
+                throw new IOException("No classworlds*.jar found in "+bootDir" -- Is this a valid maven2 directory?");
 
             boolean isMaster = getCurrentNode()==Hudson.getInstance();
             FilePath slaveRoot=null;