Add exception for catching in IO

Created by: mattpitkin

I'm attempting to use TimeSeriesBase class as the basis for a data object in my new CWInPy code. I've written a new IO reader registry for the data format. However, an allowed data format for reading is an acsii txt file that has four columns. In these cases when the io.is_cache function checks these sort of files it causes a RuntimeError exception. However, this is not an exception that is caught, so this is_cache function doesn't just exit as False, but instead fails with an exception. This PR just adds RuntimeError as an exception that means that is_cache does return False in this case.