Add class errorbox to showError()

This one-liner gives errors a nicer default appearance, using the CSS in core
(skins/common/shared.css).  Still TODO is adding some spacing around the
div otherwise it bumps up against the buttons, see
http://imgur.com/ukSglrJ

One way to trigger an error is turn off Parsoid, add a topic, click [Save]

Change-Id: I94a3bd73f579272cf9ca0ce04b9e95625f9b0a31
Merge "Add class errorbox to showError()"