Add parsing errors in lua backend

Summary: Luabackend haven't parsed errors, so if entries cause errors, we just get empty output (but not empty error output, which don't check). This add error handling, so now we could see, if errors hapened.

Test Plan:
1) Start lua session, and check, that wrong lua expression don't produce any output.
2) Apply this patch
3) Check, that how wrong lua expression produce output and cause error entry status

Reviewers: #cantor, asemke

Subscribers: #cantor, #kde_edu

Tags: #kde_edu, #cantor

Differential Revision: https://phabricator.kde.org/D11963