json: Add methods to wait for cx, endp to be found.

This lets scripts wait until we are sure the system can handle
further work.

Ensure we print out error in case the URL processing logic
detects and error and plans to exit.

py-json/LANforge/lfcli_base.py

@@ -196,7 +196,7 @@ class LFCliBase:
     # use this inside the class to log a failure result
     def _fail(self, message, print_=False):
         self.test_results.append(self.fail_pref + message)
-        if print_:
+        if print_ or self.exit_on_fail:
             print(self.fail_pref + message)
         if self.exit_on_fail:
             sys.exit(1)