Add a custom exception when the model path is not correct

Signed-off-by: Idlir Shkurti <[email protected]>

Author: idlir-shkurti-frequenz

src/frequenz/sdk/ml/_model_manager.py

@@ -28,6 +28,18 @@ class _Model(Generic[T]):
     path: Path
 
 
+class ModelNotFoundError(Exception):
+    """Exception raised when a model is not found."""
+
+    def __init__(self, key: str) -> None:
+        """Initialize the exception with the specified model key.
+
+        Args:
+            key: The key of the model that was not found.
+        """
+        super().__init__(f"Model with key '{key}' is not found.")
+
+
 class ModelManager(BackgroundService, Generic[T]):
     """Load, update, monitor and retrieve machine learning models."""
 
@@ -59,12 +71,13 @@ def _load(path: Path) -> T:
             T: The loaded model data.
 
         Raises:
-            FileNotFoundError: If the model file does not exist.
+            ModelNotFoundError: If the model file does not exist.
         """
-        if not path.exists():
-            raise FileNotFoundError(f"The model path {path} does not exist.")
-        with path.open("rb") as file:
-            return cast(T, pickle.load(file))
+        try:
+            with path.open("rb") as file:
+                return cast(T, pickle.load(file))
+        except FileNotFoundError as exc:
+            raise ModelNotFoundError(str(path)) from exc
 
     @override
     def start(self) -> None: