This fixes the scenario where a note file referenced by a todo, event,
or appointment is modified from outside the program in such a way that
it cannot be opened by the program, resulting in a segmentation fault.
The least surprising way to proceed is to ignore the note.
Signed-off-by: Nicholas Johnson <nick@nicksphere.ch>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>
