Fix wrong message displayed in case of a use of the journey planner service with wrong date format

For example, when trying to use a service like this: http://cgn-dev.virt.moviplus.ch:8080/?SERVICE=journey_planner&da=2018-09-27, an error, which not really understandable, is returned:

500 Internal Server Error basic_string::at: __n (which is 0) >= this->size() (which is 0)