Opening a file with an encoding other than UTF-8 fails silently

[PatchMixolydic]

While testing cosmic-edit, I tried opening a Shift-JIS encoded document. I expected to see either an error message or the document's content. Instead, I was presented with a new (blank) document.

cosmic-edit did write an error message to my terminal:

[2024-07-01T01:53:15Z ERROR cosmic_edit::tab] failed to open "/home/patchmixolydic/Documents/shift-jis.txt": stream did not contain valid UTF-8

However, this would not be visible to a typical end user. It would be helpful if a message box could be displayed to explain that UTF-8 is the only supported encoding (at least for now).

Tested on e64bdac.