Improve error message for config file.
Improve error message for config file.

tribut/vdirsyncer,hobarrera/vdirsyncer,credativUK/vdirsyncer,untitaker/vdirsyncer,hobarrera/vdirsyncer,tribut/vdirsyncer,mathstuf/vdirsyncer,untitaker/vdirsyncer,credativUK/vdirsyncer,untitaker/vdirsyncer,mathstuf/vdirsyncer