controller: update status schema

doc/controller/Protocol.md

@@ -173,8 +173,11 @@ After receiving such a message, each entity shall send a status update message a
 		"state" : "running",
 		"version" : "0.1.0",                 /* VILLAScontroller version */
 		"uptime" : 123124.0,                 /* in seconds since initialization (float) */
-		"result": "XXXXXXX"                  /* only if state == "running" */
-		"error": "This is an error message", /* only if state == "error" */
+
+		"error": {                           /* only present if state == "error" */
+			"message": "This is an error message",
+			"code": 1234
+		},
 
 		"managed_by": "4af1cf3a-7e80-11eb-a7f4-b700f4a4f18d", /* link to managing IC of category 'manager' */