Avoid previous content lost in case of disk full

[nplanel]

to reproduce

• add some stuff
• fill up the partitions
• reconnect to the server
• edit an article
  if you sync or online the new content is not saved and the previous one is lost, an empty page is returned, saved on disk

Would be nice to handle the "write" error by creating a tempfile then rename it (like vim does on save file)
This would avoid to overwrite the previous content to an empty page