add custom exception

ntfy/exceptions.py

@@ -0,0 +1,8 @@
+class SubscriptionError(Exception):
+    """Raised when an error occurs while subscribing to a ntfy topic."""
+
+    def __init__(self, http_response: dict) -> None:
+        self._http_message = http_response
+        self.message = self._http_message.get("error")
+        self.code = self._http_message.get("code")
+        super().__init__(self.message)