When retrieving user_data parameters and readback path doesn't match, generate a more user-friendly error message #30
Description
When reading a user_data parameter, if the parameter is not set the API will respond with an empty string, and path=$DETECTOR_NAME/user_data. The exception message should state that a user_data parameter was requested that is not set in this case (it is less of a critical error than if a non-user-data parameter could not be read).