Don't call AbstractRevision::getContent unless when needed

Also changing the post content to an error message when it failed to load from
Parsoid.

As suggested in https://gerrit.wikimedia.org/r/#/c/127666/5/includes/Templating.php

Change-Id: I70f86fec30237e00e4126ff0c50c8328de814c8c
Merge "Don't call AbstractRevision::getContent unless when needed"