Thanks for your bugreport.
This sounds like the mirror (fr.archive.ubuntu.com) was overloaded.
Could you please try the upgrade again and see if it works better than?
** Changed in: update-manager (Ubuntu)
Status: New => Incomplete
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7.10 -> 8.04 Failed to fetch fr.archive.ubuntu.com
https://bugs.launchpad.net/bugs/223691
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