try to backport retries for failed files

[ThomasWaldmann]

see #7351.

[ThomasWaldmann]

I will have a look at this (again).

IIRC, I did not backport it back then, because it is only that easy in master branch (borg2), which works significantly different than borg 1.x does:

• borg 1.x archives .partN files when it checkpoints in the middle of a file