UX: notify user about filesystem errors

[infinity0]

I am using syncthing between 2 Debian devices and 2 Android devices. At some point, sync between them hung at 99%. There was no obvious error indicator to the user why this was happening.

By chance, I realised that on Debian filesystems, most characters are valid; whereas on Android, many characters such as : were invalid. Renaming the file on the Debian system to drop a : make the sync go through.

This is a normal non-technical error that can happen to any user. Therefore the error message should be exposed in the UX to tell the user what's wrong, and how they can fix it. Even if the OS does not exactly say "filename contains invalid character", a generic "could not create file: " is better than no error message at all.

[severen]

I ran into this issue today. It took some poking around in the (somewhat hidden) web GUI in the app to figure out the issue, and even then it was a non-obvious 'permission denied' error which took further Googling to diagnose.

I would argue that this is something Syncthing should have an option to automatically correct for (syncthing/syncthing#1734, syncthing/syncthing#7876), but either way, a clearer warning in the app would be good, especially for less technical users.

For reference, the related issue on Syncthing's side is syncthing/syncthing#8451.