`borg extract` should retry failed files (up to X times) instead of failing the entire job

When a file fails checksum verification during `borg extract` many times this is simply due to a network error, and can be fixed by retrying the file. When I'm restoring 25 TB of files, I don't want one random failure to force me to restart the entire job, especially since there is no [deduplication mechanism (ala rsync)](https://github.com/borgbackup/borg/issues/95) in `borg extract`.

To me, this is a critical issue, because `borg` is unable to perform the primary job that I expect of it: restoring my files from backup. Instead I have to do a tedious workaround of getting a list of all files that need restored, and doing them one directory at a time, when there are hundreds of directories.