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/LFRequest.py

@@ -137,7 +137,7 @@ class LFRequest:
                 response_json_list_.append(j)
             return responses[0]
         except urllib.error.HTTPError as error:
-            if show_error:
+            if show_error or die_on_error_ or (error.code != 404):
                 print("----- LFRequest::jsonPost:138 HTTPError: --------------------------------------------")
                 print("<%s> HTTP %s: %s"%(request.get_full_url(), error.code, error.reason, ))