sync: fix --cutoff-mode soft & cautious so it doesn't end the transfer early

Before ths fix --cutoff-mode soft and cautious would emit a Fatal
error which stopped the sync immediately.

This fix introduces a new error which is checked in the sync error
processing which stops the sync gracefully.

Fixes #4576

fs/accounting/accounting.go

@@ -26,6 +26,10 @@ var ErrorMaxTransferLimitReached = errors.New("Max transfer limit reached as set
 // transfer limit is reached.
 var ErrorMaxTransferLimitReachedFatal = fserrors.FatalError(ErrorMaxTransferLimitReached)
 
+// ErrorMaxTransferLimitReachedGraceful is returned from operations.Copy when the max
+// transfer limit is reached and a graceful stop is required.
+var ErrorMaxTransferLimitReachedGraceful = fserrors.NoRetryError(ErrorMaxTransferLimitReached)
+
 // Account limits and accounts for one transfer
 type Account struct {
 	stats *StatsInfo