Only set display_name in OrderPopulator#check_stock_level when it's needed

This should speed up in_stock query checking, as it's no longer querying option_values + option_types tables as well.
Move Error Text calculation inside error block
Fixed bad style escaping within admin/products/index.html.erb

Fixes #3439
Do not check stock quantity twice when saving line item

Every time an user pressed the add to cart button spree was checking
stock in OrderPopulator and before saving the line item, through
Stock::AvailabilityValidator. I believe we only need to check stock on
the LineItem model

Fixes #3434

Conflicts:
	core/app/models/spree/order_populator.rb