Generic input/output error is returned when checking for the existence of a file

[GaijinSystems]

In reverse encryption mode (not tested in normal mode), there doesn't appear to be a good way to determine the existence of a file because a generic i/o error is returned, as opposed to something specific.

This was discussed/discovered in this github issue:
https://github.com/syncthing/syncthing/issues/6171#issuecomment-557257895
and still exists in a binary built from mater.