Use reference strings in Griddy (#41079)

homeassistant/components/griddy/strings.json

 {
   "config": {
     "error": {
-      "cannot_connect": "Failed to connect, please try again",
-      "unknown": "Unexpected error"
+      "cannot_connect": "[%key:common::config_flow::error::cannot_connect%]",
+      "unknown": "[%key:common::config_flow::error::unknown%]"
     },
     "step": {
       "user": {