Collect unprinted Ign errors for display in Err output

Instead of deciding if the first or the last error in a retry chain is
the best one to display if the entire chain fails in the end we teach
our progress reporting to collect the ignored errors (if it hasn't shown
them) and print them all together in the Err line.

We de-duplicate the collection which makes it harder to match the Ign
line with the error messages, but for these cases its probably better to
enable direct display of the text anyhow – and repeating the same message
multiple times is boring and probably not that helpful for a user
wanting to know why all the downloads failed even they asked apt to
retry 429 times…

References: 841243df877b39dfcb907a7fab9c4e8b4f65d715

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