fix: improve error messaging and handling in get_provider_models

- Updated error message formatting in `get_provider_models` call within `Backend_Api` class
- Changed `MissingAuthError` handling to include exception type name in response
- Added generic `Exception` catch to handle unexpected errors with HTTP 500 response
- Modified `backend_api.py` file in `g4f/gui/server` directory
- Ensured all returned error messages use consistent structure with exception type and message

g4f/gui/server/backend_api.py

@@ -123,7 +123,9 @@ class Backend_Api(Api):
             try:
                 response = self.get_provider_models(**kwargs)
             except MissingAuthError as e:
-                return jsonify({"error": {"message": str(e)}}), 401
+                return jsonify({"error": {"message": f"{type(e).__name__}: {e}"}}), 401
+            except Exception as e:
+                return jsonify({"error": {"message": f"{type(e).__name__}: {e}"}}), 500
             return jsonify(response)
 
         @app.route('/backend-api/v2/providers', methods=['GET'])