update_distfiles: report errors properly
update_distfiles: report errors properly

If files cannot be mirrored due to problems fetching or missing digests
report that loudly so we are actually aware of it and can fix it.

mjg59/scripts,smilart/scripts,mjg59/scripts,smilart/scripts,coreos/scripts,glevand/coreos--scripts,glevand/coreos--scripts,marineam/coreos-scripts,marineam/coreos-scripts,mjg59/scripts,coreos/scripts,marineam/coreos-scripts,smilart/scripts