calcurse-upgrade: Display error if temp file exists
calcurse-upgrade: Display error if temp file exists

Instead of bailing out without any hint, show an error message if the
temporary file we use in calcurse-upgrade already exists.

Signed-off-by: Lukas Fleischer <af90e31281d8305af43dc616c5aaca3c7165f8a7@cryptocrack.de>

lfos/calcurse,lfos/calcurse,lfos/calcurse