Downloads should attempt all URLs in the repository before failing

Created by: Zaharid

It so happens that the hepforge repository is frequently malfunctioning. For example, currently trying to download a PDF set from here: https://www.hepforge.org/downloads/lhapdf/pdfsets/6.1/NNPDF31_nlo_as_0118.tar.gz

results in a generic html being downloaded and then the decompression failing. This in turn breaks the tests and everything else.

It would be good if we could have subsequent repositories attempting the download if the previous one failes, e.g. for a PDF set that is in all the repositories.