Use common strings in Mill config flow (#41836)

homeassistant/components/mill/config_flow.py

@@ -40,7 +40,7 @@ class MillConfigFlow(config_entries.ConfigFlow, domain=DOMAIN):
         errors = {}
 
         if not await mill_data_connection.connect():
-            errors["connection_error"] = "connection_error"
+            errors["cannot_connect"] = "cannot_connect"
             return self.async_show_form(
                 step_id="user",
                 data_schema=DATA_SCHEMA,

homeassistant/components/mill/strings.json

@@ -4,7 +4,7 @@
       "already_configured": "[%key:common::config_flow::abort::already_configured_account%]"
     },
     "error": {
-      "connection_error": "[%key:common::config_flow::error::cannot_connect%]"
+      "cannot_connect": "[%key:common::config_flow::error::cannot_connect%]"
     },
     "step": {
       "user": {

tests/components/mill/test_config_flow.py

@@ -87,4 +87,4 @@ async def test_connection_error(hass):
         )
 
     assert result["type"] == "form"
-    assert result["errors"]["connection_error"] == "connection_error"
+    assert result["errors"]["cannot_connect"] == "cannot_connect"