cmd: add --error-on-no-transfer option

This allows rclone to exit with a non-zero return code if no files are
transferred. This is useful when calling rclone as part of a workflow/script
pipeline as it allows the end user to stop processing if no files have been
transferred.

NB: Enabling this option will return in rclone exiting non-zero if there are no
transfers. Depending on how your're currently using rclone in your scripts,
this may break existing setups!

cmd/cmd.go

@@ -66,6 +66,7 @@ const (
 	exitCodeNoRetryError
 	exitCodeFatalError
 	exitCodeTransferExceeded
+	exitCodeNoFilesTransferred
 )
 
 // ShowVersion prints the version to stdout
@@ -312,6 +313,7 @@ func Run(Retry bool, showStats bool, cmd *cobra.Command, f func() error) {
 		}
 	}
 	resolveExitCode(cmdErr)
+
 }
 
 // CheckArgs checks there are enough arguments and prints a message if not
@@ -430,6 +432,11 @@ func initConfig() {
 func resolveExitCode(err error) {
 	atexit.Run()
 	if err == nil {
+		if fs.Config.ErrorOnNoTransfer {
+			if accounting.GlobalStats().GetTransfers() == 0 {
+				os.Exit(exitCodeNoFilesTransferred)
+			}
+		}
 		os.Exit(exitCodeSuccess)
 	}