Bail out when reading dates such as "02/30/2013" from the appointments
file. These *could* be converted into valid dates but since we never
write invalid dates to that file, these indicate a user error.
Fixes following test cases:
* appointment-009.sh
* appointment-012.sh
* appointment-016.sh
* appointment-019.sh
* event-003.sh
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>