Bugfix: Do not return an error when the scanner is slower than backup

When restic makes a backup, there's a background task called "scanner" which
collects information on how many files and directories are to be saved, in
order to display progress information to the user. When the backup finishes
faster than the scanner, it is aborted because the result is not needed any
more. This logic contained a bug, where quitting the scanner process was
treated as an error, and caused restic to print an unhelpful error message
("context canceled").

https://github.com/restic/restic/issues/1978
https://github.com/restic/restic/pull/1991
