Improved error message of failed downloads: List each package only once

Usually when a non-deb file (Index file) failed to download other
possible URLs with file extensions like .gz, .lzma, .bz (or no
extension at all) will get tried. For each of them a single error
line will get generated.

This patch takes care that for every failed Index/Package only one
error URLs is generated containing thei tried file suffixes.