Better error messages when not passing required arguments for specific filetypes to generic read method

[bhazelton]

This is a particular problem for votable files, where a number of column names must be specified. If you call the read_votable_file method, they are positional, required inputs. But when you call the generic read function they are defaulted to None and then you get a fairly unhelpful error message because it's trying to do work on a None object. These Nones should be caught in either the generic read method or the read_votable_file method (probably the latter).