Use new error-block class for error messages

At the moment, error messages get the class help-block. However the same
help-block class is applied to help text. It makes it hard to style
errors and help messages differently.

Work done by @timjacobi in #998. I was skeptical of all the CSS changes
and hence re-did the CSS compilation. I also only pulled across the
class changes and ignored some of the other HTML changes as they've
since been applied on master.

Fixes #998

Conflicts:
	oscar/templates/oscar/basket/partials/basket_content.html
	oscar/templates/oscar/dashboard/catalogue/product_update.html