Highlighting indexer: Enable errors

Summary:
Erros include:

• missing contexts
• duplicate contexts
• missing itemDatas
• missing keyword lists
• duplicate keyowrd lists

Warnings include:

• unused contexts
• unused itemDatas
• unused keyword lists

Test Plan: make test

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9377