Raise ConfigEntryNotReady for MQTT connection exception (#22540)

* Raise ConfigEntryNotReady for connection exception

Raise ConfigEntryNotReady for the connection exception like if the MQTT Server container/device is being restarted or was unavailable on boot.

* Add new exception

* grammar fix

* Possibly resolve hound comments

* raise `ConfigEntryNotReady` for mqtt connection error

* revert exceptions.py

* Update exceptions.py

* modify test to handle exception

* use constants to control exception scope

* Raise ConfigEntryNotReady for connection exception

Raise ConfigEntryNotReady for the connection exception like if the MQTT Server container/device is being restarted or was unavailable on boot.

* Add new exception

* Add new exception

* grammar fix

* Possibly resolve hound comments

* raise `ConfigEntryNotReady` for mqtt connection error

* revert exceptions.py

* Update exceptions.py

* modify test to handle exception

* use constants to control exception scope

* revert test change as it's not the same thing

* Update test_init.py

* Add test for MQTT OSError

* revert file changes from a bad rebase

* Rewrite test with valid syntax

* rewrite test to be less ambiguous

* add empty line

* add back 'axis'

* Remove empty line

* Update tests and undo merge from earlier

* correctly restore test for no connect broker

* fix test mock correctly

* line was too long. hit enter.

tests/components/mqtt/test_init.py

@@ -12,6 +12,7 @@ from homeassistant.const import (
     ATTR_DOMAIN, ATTR_SERVICE, EVENT_CALL_SERVICE, EVENT_HOMEASSISTANT_STOP)
 from homeassistant.core import callback
 from homeassistant.setup import async_setup_component
+from homeassistant.exceptions import ConfigEntryNotReady
 
 from tests.common import (
     MockConfigEntry, async_fire_mqtt_message, async_mock_mqtt_component,
@@ -621,6 +622,19 @@ async def test_setup_fails_if_no_connect_broker(hass):
         assert not await mqtt.async_setup_entry(hass, entry)
 
 
+async def test_setup_raises_ConfigEntryNotReady_if_no_connect_broker(hass):
+    """Test for setup failure if connection to broker is missing."""
+    entry = MockConfigEntry(domain=mqtt.DOMAIN, data={
+        mqtt.CONF_BROKER: 'test-broker'
+    })
+
+    with mock.patch('paho.mqtt.client.Client') as mock_client:
+        mock_client().connect = mock.Mock(
+            side_effect=OSError("Connection error"))
+        with pytest.raises(ConfigEntryNotReady):
+            await mqtt.async_setup_entry(hass, entry)
+
+
 async def test_setup_uses_certificate_on_certificate_set_to_auto(
         hass, mock_MQTT):
     """Test setup uses bundled certs when certificate is set to auto."""