Remove reliance on Flash styling within ee/app/views/trials/_errors.html.haml

The following HAML template relies on "Flash" styles for an alert-like banner:

• ee/app/views/trials/_errors.html.haml

Since Flash is intended to be refactored to utilize GlAlert, the other bits of code which rely on its styles will need to be refactored as well, either employing GlAlert or its gl-alert CSS classes and DOM structure.