Handle query prepare errors in QueryBuilder::exec

Summary:
It is way better to handle these errors here as early as possible. It
means seeing error descriptions which actually make sense (in e.g. SQLite
case, when SQLite limits or syntax errors are hit) and probably it should
help to avoid nasty crashes later on in exec() phase.

Reviewers: KDE PIM, dvratil

Reviewed By: KDE PIM, dvratil

Subscribers: dvratil, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D17877