[repo] Replace the 'Failed to synchronize cache' message (RhBug:1712055)
[repo] Replace the 'Failed to synchronize cache' message (RhBug:1712055)

The message was misleading and indicated that there's
a problem with cache rather than with downloading repodata.

rpm-software-management/libdnf,rpm-software-management/libhif,rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libhif,rpm-software-management/libdnf,rpm-software-management/libhif,hughsie/libhif,hughsie/libhif,rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libhif