Fix two bugs in installer session error display

Fix two bugs in installer session error display

If Installer::startSession() returns false, then showError() is called
before setParserLanguage(), which causes DB access via
Title::getContentModel(). The code which was meant to handle DB access
from the parser did not do so, since the exception class which is thrown
on attempted DB access has changed.

Change-Id: I1478ec06a441851c5336bdde15734642615d45e9