Fix error handling on subscribe when mqtt is not initialized

[jbouwh]

Proposed change

When an automation trigger is set up, and it tries to subscribe an MQTT topic, while MQTT was not inialized correctly (e.g. becase of a yaml configuration failure), mqtt will throw an unhandled KeyError exception like:

2023-10-11 15:41:37.812 DEBUG (MainThread) [homeassistant.components.mqtt.trigger] Attaching MQTT trigger for topic: 'test/trigger', payload: 'now'
2023-10-11 15:41:37.848 INFO (MainThread) [homeassistant.components.automation.test_trigger] Initialized trigger Test trigger
2023-10-11 15:41:37.850 ERROR (MainThread) [homeassistant.components.automation.test_mqtt_trigger] Error setting up trigger test_mqtt_trigger
Traceback (most recent call last):
  File "/workspaces/core/homeassistant/components/mqtt/trigger.py", line 101, in async_attach_trigger
    remove = await mqtt.async_subscribe(
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/core/homeassistant/components/mqtt/client.py", line 179, in async_subscribe
    mqtt_data = get_mqtt_data(hass)
                ^^^^^^^^^^^^^^^^^^^
  File "/workspaces/core/homeassistant/components/mqtt/util.py", line 180, in get_mqtt_data
    mqtt_data = hass.data[DATA_MQTT]
                ~~~~~~~~~^^^^^^^^^^^
KeyError: 'mqtt'
2023-10-11 15:41:37.856 INFO (MainThread) [homeassistant.components.automation.test_98664] Initialized trigger Test #98664

This PR makes sure this type of error is handled correctly, so we get something like:

2023-10-11 15:39:08.530 ERROR (MainThread) [homeassistant.components.automation.test_mqtt_trigger] Got error 'Unable to subscribe MQTT topic test/trigger. Makes sure the MQTT integration is set up correctly' when setting up triggers for test_mqtt_trigger

get_mqtt_data is used to expose an internal dataclass, to ensure typing, and it is set when mqtt is initialized the first time.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
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• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
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• This PR fixes or closes issue: fixes #
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[home-assistant]

Hey there @emontnemery, mind taking a look at this pull request as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!

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