backend/retry: return worker function error and abort

This is a bug fix: Before, when the worker function fn in List() of the
RetryBackend returned an error, the operation is retried with the next
file. This is not consistent with the documentation, the intention was
that when fn returns an error, this is passed on to the caller and the
List() operation is aborted. Only errors happening on the underlying
backend are retried.

The error leads to restic ignoring exclusive locks that are present in
the repo, so it may happen that a new backup is written which references
data that is going to be removed by a concurrently running `prune`
operation.

The bug was reported by a user here:
https://forum.restic.net/t/restic-backup-returns-0-exit-code-when-already-locked/484

internal/restic/lock.go

@@ -44,7 +44,11 @@ type ErrAlreadyLocked struct {
 }
 
 func (e ErrAlreadyLocked) Error() string {
-	return fmt.Sprintf("repository is already locked by %v", e.otherLock)
+	s := ""
+	if e.otherLock.Exclusive {
+		s = "exclusively "
+	}
+	return fmt.Sprintf("repository is already locked %sby %v", s, e.otherLock)
 }
 
 // IsAlreadyLocked returns true iff err is an instance of ErrAlreadyLocked.