contrip/upload: do not fail sftp if directory already exists

contrib/upload

@@ -25,9 +25,12 @@ else
 fi
 
 
+# do not fail sftp if directory exists
+# see https://stackoverflow.com/questions/51437924/bash-shell-sftp-check-if-directory-exists-before-creating
+
 sftp -oBatchMode=no -b - "$SSHUSER@uploadserver" << !
    cd electrum-downloads-airlock
-   mkdir "$version"
+   -mkdir "$version"
    cd "$version"
    mput *
    bye