Improve error messages on wrong storage params
Improve error messages on wrong storage params

Fix #38

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