Use common strings in opentherm_gw config flow (#42211)

* Use common strings in opentherm_gw config flow

* Combine TimeourError and SerialException handling

* Fixed test_form_connection_error

* And then fix the flake8 error...

* Change "timeout" to "cannot_connect" msg

tests/components/opentherm_gw/test_config_flow.py

@@ -148,7 +148,7 @@ async def test_form_connection_timeout(hass):
         )
 
     assert result2["type"] == "form"
-    assert result2["errors"] == {"base": "timeout"}
+    assert result2["errors"] == {"base": "cannot_connect"}
     assert len(mock_connect.mock_calls) == 1
 
 
@@ -164,7 +164,7 @@ async def test_form_connection_error(hass):
         )
 
     assert result2["type"] == "form"
-    assert result2["errors"] == {"base": "serial_error"}
+    assert result2["errors"] == {"base": "cannot_connect"}
     assert len(mock_connect.mock_calls) == 1