More informative error server error message.

bin/HFO

@@ -65,6 +65,8 @@ def main(args):
     time.sleep(0.2)
     assert server.poll() is None,\
       '[start.py] Failed to launch Server with command: \"%s\"' \
+      '\n\nAnother rcssserver may be running on the same port?' \
+      '\nTry: \"killall -9 rcssserver\"' \
       %(serverCommand + serverOptions)
     if not args.headless:
       monitorCommand = os.path.join(binary_dir, MONITOR_BIN)