Warn if backup failed to read tree blob

changelog/unreleased/pull-2978

@@ -0,0 +1,8 @@
+Enhancement: Warn if parent snapshot cannot be loaded during backup
+
+During a backup restic uses the parent snapshot to check whether a file was
+changed and has to be backed up again. For this check the backup has to read
+the directories contained in the old snapshot. If a tree blob cannot be
+loaded, restic now warns about this problem with the backup repository.
+
+https://github.com/restic/restic/pull/2978