check existence of all input files for summarypages at the start

When running summarypages on a lot of large --samples files, it can be annoying to have it run for a good while on the first few files, then fail when one of the paths had a typo. By now I've learned to do a ll on the input list first, but it would be a nice convenience feature if summarypages could do a quick check that all inputs exist before starting to loop over them.