Fix unittests for 3.1rc
Merge pull request #25 from stojg/fix-tests-for-31

Fix unittests for 3.1rc
NEW Generate static versions of ErrorPages.

The error-XXX.html pages generated in assets folder use the dynamic URL
so they can't be used on the frontend. Provide a copy of these in cache
folder during the publisher processing task.
Merge pull request #24 from mateusz/standard-error-pages

NEW Generate static versions of ErrorPages.
Update buildcache url in documentation (Fixes: #23)