improving an error diagnostics for a plugin developer

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@17811 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/model/Slave.java

@@ -423,7 +423,10 @@ public abstract class Slave extends Node implements Serializable {
     }
 
     public SlaveDescriptor getDescriptor() {
-        return (SlaveDescriptor)Hudson.getInstance().getDescriptor(getClass());
+        Descriptor d = Hudson.getInstance().getDescriptor(getClass());
+        if (d instanceof SlaveDescriptor)
+            return (SlaveDescriptor) d;
+        throw new IllegalStateException(d.getClass()+" needs to extend from SlaveDescriptor");
     }
 
     public static abstract class SlaveDescriptor extends NodeDescriptor {