fix: check local path exists

See coop-cloud/organising#245.

cli/app/cp.go

@@ -76,6 +76,12 @@ And if you want to copy that file back to your current working directory locally
 			logrus.Debugf("assuming transfer is going TO the container")
 		}
 
+		if !isToContainer {
+			if _, err := os.Stat(dstPath); os.IsNotExist(err) {
+				logrus.Fatalf("%s does not exist locally?", dstPath)
+			}
+		}
+
 		appFiles, err := config.LoadAppFiles("")
 		if err != nil {
 			logrus.Fatal(err)