Style warnings from live preview on Special:Upload

The live preview about warnings on Special:Upload does not style
warnings added to the form. Added styles to the container to style the
warnings.
Also reuse the style to give the warning box on the non-js from the same
style. The visual effect here is an expand box to the full width.

Change-Id: I36e0bf0332eac476002569f46ab116f271f3c468