sftp: ignore errors when closing the connection pool

There is no need to report errors when draining the connection pool -
they are useless at this point.

See: https://forum.rclone.org/t/rclone-fails-to-close-unused-tcp-connections-due-to-use-of-closed-network-connection/46735

backend/sftp/sftp.go

@@ -815,13 +815,13 @@ func (f *Fs) drainPool(ctx context.Context) (err error) {
 		if cErr := c.closed(); cErr == nil {
 			cErr = c.close()
 			if cErr != nil {
-				err = cErr
+				fs.Debugf(f, "Ignoring error closing connection: %v", cErr)
 			}
 		}
 		f.pool[i] = nil
 	}
 	f.pool = nil
-	return err
+	return nil
 }
 
 // NewFs creates a new Fs object from the name and root. It connects to