Use reference strings for Nut configflow (#40966)

homeassistant/components/nut/strings.json

@@ -25,11 +25,11 @@
       }
     },
     "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%]"
     },
     "abort": {
-      "already_configured": "Device is already configured"
+      "already_configured": "[%key:common::config_flow::abort::already_configured_device%]"
     }
   },
   "options": {
@@ -43,4 +43,4 @@
       }
     }
   }
-}
+}