No message to user that saving fails when disk is full

I tried to save a file and thought it was saved, since i got no message that saving failed. So i closed the editor, which lead to data loss!

As i started code from console, i got this errors:

```
(io.elementary.code:7445): dconf-WARNING **: 16:50:32.099: failed to commit changes to dconf: GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._g_2dfile_2derror_2dquark.Code12: Failed to write file “/home/davidak/.config/dconf/user.6ICC5Z”: write() failed: No space left on device
[WARNING 16:50:40.188720] Document.vala:834: Cannot create backup copy for file "configuration.nix": Error splicing file: No space left on device
[WARNING 16:50:40.197186] Document.vala:477: Cannot save "configuration.nix": Error writing to file: No space left on device
[WARNING 16:50:42.616458] Document.vala:477: Cannot save "configuration.nix": Error writing to file: No space left on device
```

Please also show it to users of the GUI and warn them to close the editor in such a case!

---

Code 3.1.1
on NixOS 19.03.173165.55b8860aa20