Replace ValueError with info attribute with SlycotError

[roryyorke]

See #84 (comment)

A common pattern in the Slycot code is

        e = ValueError('the number of eigenvalues to be assigned is less than the number of possibly assignable eigenvalues')
        e.info = info
        raise e

The idea is to have a SlycotError class, so that this can be:

    raise SlycotError(msg, info)

instead - that way one can't forget the .info attribute.

I'd have had SlycotError inherit from RuntimeError, but for backward compatibility it should probably be ValueError.