Bugfix: Complete intermediate index upload

After a user posted a comprehensive report of what he observed, we were able to
find a bug and correct it: During backup, restic uploads so-called
"intermediate" index files. When the backup finishes during a transfer of such
an intermediate index, the upload is cancelled, but the backup is finished
without an error. This leads to an inconsistent state, where the snapshot
references data that is contained in the repo, but is not referenced in any
index.

The situation can be resolved by building a new index with `rebuild-index`, but
looks very confusing at first. Since all the data got uploaded to the repo
successfully, there was no risk of data loss, just minor inconvenience for our
users.

https://github.com/restic/restic/pull/1589
https://forum.restic.net/t/error-loading-tree-check-prune-and-forget-gives-error-b2-backend/406