Add error message when filename contains illegal characters for target storage

On Linux, ? and : are valid characters for filenames. On Android, they are not. As a result, the files cannot be written. My current setup: syncing files between my Linux laptop and Android phone. I tried to sync files that had :'s and ?'s in their names. The sync failed without giving useful error codes.

Current error messages:

In Linux web GUI (just shows the sync hanging)

Android app (Error message: "Out of Sync")

Android web GUI

(Error message: "syncing: finishing: opening temp file: open [PATH]/.syncthing.[FILENAME].tmp: operation not permitted"

Proposed warning: "Caution: Filenames contain characters that are not allowed on some operating systems."