Generate HomeAssistantError message from English translations (#113305)

* Fetch exception message from translation cache

* Improve tests

* Return translation key without path, cleanup

* Fetch translations when string variant is requested

* Move import

* revert changes ConfigValidationError

* mypy

* Remove _str__ method instead

* Type _message for mqtt template exception classes

* Revert changes made to test_config.py

* Undo changes TemplateError

* Follow up comments and test coverage

homeassistant/exceptions.py

@@ -13,6 +13,9 @@ if TYPE_CHECKING:
 class HomeAssistantError(Exception):
     """General Home Assistant exception occurred."""
 
+    _message: str | None = None
+    generate_message: bool = False
+
     def __init__(
         self,
         *args: object,
@@ -21,11 +24,42 @@ class HomeAssistantError(Exception):
         translation_placeholders: dict[str, str] | None = None,
     ) -> None:
         """Initialize exception."""
+        if not args and translation_key and translation_domain:
+            self.generate_message = True
+            args = (translation_key,)
+
         super().__init__(*args)
         self.translation_domain = translation_domain
         self.translation_key = translation_key
         self.translation_placeholders = translation_placeholders
 
+    def __str__(self) -> str:
+        """Return exception message.
+
+        If no message was passed to `__init__`, the exception message is generated from
+        the translation_key. The message will be in English, regardless of the configured
+        language.
+        """
+
+        if self._message:
+            return self._message
+
+        if not self.generate_message:
+            self._message = super().__str__()
+            return self._message
+
+        if TYPE_CHECKING:
+            assert self.translation_key is not None
+            assert self.translation_domain is not None
+
+        # pylint: disable-next=import-outside-toplevel
+        from .helpers.translation import async_get_exception_message
+
+        self._message = async_get_exception_message(
+            self.translation_domain, self.translation_key, self.translation_placeholders
+        )
+        return self._message
+
 
 class ConfigValidationError(HomeAssistantError, ExceptionGroup[Exception]):
     """A validation exception occurred when validating the configuration."""
@@ -47,10 +81,6 @@ class ConfigValidationError(HomeAssistantError, ExceptionGroup[Exception]):
         )
         self._message = message
 
-    def __str__(self) -> str:
-        """Return exception message string."""
-        return self._message
-
 
 class ServiceValidationError(HomeAssistantError):
     """A validation exception occurred when calling a service."""