move json patch validation to a separate file

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
dbdict does not need to import JSON Pointer

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
fix model data dumping

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
stop converting common exceptions to API errors

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
reworked patch validation and error reporting

Signed-off-by: Jan Dvořák <mordae@anilinux.org>