feat(shared): add kernel exceptions

backend/app/shared/kernel/exceptions.py

@@ -0,0 +1,18 @@
+class NotFoundError(Exception):
+    def __init__(self, entity: str, entity_id: str) -> None:
+        self.entity = entity
+        self.entity_id = entity_id
+        super().__init__(f"{entity} not found: {entity_id}")
+
+
+class ValidationError(Exception):
+    def __init__(self, message: str) -> None:
+        self.message = message
+        super().__init__(message)
+
+
+class FileSystemError(Exception):
+    def __init__(self, path: str, message: str) -> None:
+        self.path = path
+        self.message = message
+        super().__init__(f"Filesystem error at {path}: {message}")