Attempt download from private repo should lhapdf download fail

Created by: wilsonmr

Aim to introduce new fallback logic so that if a download/extract fails from lhapdf/hepforge url, private repos are checked before an error is raised.

Closes #257 (closed)

Still actually need to test this on a pathological URL, today the current and old LHAPDF urls appear be working for NNPDF31_nnlo_as_0118. Have no idea if the except: return structure I proposed will actually do the job