Write custom error class for SALib

[willu47]

At present, RuntimeErrors are used within each of the analysis modules. However, the text that describes the errors is inconsistent between the different methods (although sometimes needs to be different).

A custom SALib error class would allow us to maintain a focussed control on types of errors and the content error messages, ultimately resulting in a more informed user experience!

For example, from the Sobol module:

   if calc_second_order and Y.size % (2 * D + 2) == 0:
        N = int(Y.size / (2 * D + 2))
    elif not calc_second_order and Y.size % (D + 2) == 0:
        N = int(Y.size / (D + 2))
    else:
        raise RuntimeError("""
        Incorrect number of samples in model output file. 
        Confirm that calc_second_order matches option used during sampling.""")

    if conf_level < 0 or conf_level > 1:
        raise RuntimeError("Confidence level must be between 0-1.")

This would be replaced by:

   if calc_second_order and Y.size % (2 * D + 2) == 0:
        N = int(Y.size / (2 * D + 2))
    elif not calc_second_order and Y.size % (D + 2) == 0:
        N = int(Y.size / (D + 2))
    else:
        raise SampleError()

    if conf_level < 0 or conf_level > 1:
        raise ConfidenceIntervalError()