don't backup non-existent directories

src/freedombone

 }
 
 function backup_directory_to_usb {
+  if [ ! -d ${1} ]; then
+      echo "WARNING: directory does not exist: ${1}"
+      return
+  fi
+
   if [[ $BACKUP_TYPE == 'obnam' ]]; then
       BACKUP_KEY_EXISTS=$(gpg --list-keys "$MY_NAME (backup key)")
       if [ ! "$?" = "0" ]; then