[Feature] Add visual incicator for an unsuccessful save

[Dirleye]

Cosmic-edit version:
1.0.0.alpha.4.r0.g86bcadb-1

Issue/Bug description:
An unsuccessful attempt to save a file is not reported outside of the terminal. The most common error case is attempting to save a file with incorrect permissions.
Currently the save dialog closes as if it were successful upon a manual save, and the save-on-exit dialog repeats until the user selects to discard any changes.

Steps to reproduce:
Attempt to save any root owned file (/etc/environment).

Expected behavior:
App briefly explains that the file could not be saved and why.
App could also warn a user that the selected file is unwritable when it's opened.

Other notes: