Translate exception messages

This is a Gold rule. In quality_scale.yaml, it is called exception-translations.

Exceptions that reach the user are translatable. Raise HomeAssistantError, or a subclass like ServiceValidationError, with a translation_domain and a translation_key instead of a hardcoded English message, and put the message in the exceptions section of strings.json.

When an action fails, the user sees the error message in the interface. For a user who runs Home Assistant in their own language, an English error in the middle of a translated interface is harder to understand, exactly at the moment they need to understand what went wrong.

Exceptions that inherit from HomeAssistantError support translations out of the box, so all it takes is a key and a string.

async def async_set_schedule(call: ServiceCall) -> None:
    """Set the schedule for a day."""
    start = call.data[ATTR_START_DATE]
    end = call.data[ATTR_END_DATE]
    if end < start:
        raise ServiceValidationError(
            translation_domain=DOMAIN,
            translation_key="end_before_start",
        )

    try:
        await client.set_schedule(start, end)
    except MyConnectionError as err:
        raise HomeAssistantError(
            translation_domain=DOMAIN,
            translation_key="cannot_connect",
            translation_placeholders={"error": str(err)},
        ) from err

strings.json:

{
  "exceptions": {
    "end_before_start": {
      "message": "The end date can't be before the start date."
    },
    "cannot_connect": {
      "message": "Could not connect to the device: {error}"
    }
  }
}

Don't pass a message as well: with a translation key, the message comes from strings.json.

• Pylint, when the rule is done: raising a HomeAssistantError or a subclass with a hardcoded message is flagged (home-assistant-exception-not-translated).
• Pylint, for every integration: the translation key must exist in the exceptions section of strings.json (home-assistant-exception-translation-key-missing), translation_domain and translation_key must be set together (home-assistant-exception-translation-key-domain-mismatch), the placeholders must match the message (home-assistant-exception-placeholder-mismatch), and there must be no message next to a translation key (home-assistant-exception-message-with-translation).
• The tests fail when an action raises an exception whose translation is missing.

If the integration does not raise any exceptions of its own, this rule does not apply.

• entity-translations: translate entity names.
• action-exceptions: raise exceptions when actions fail.