API - Fix missing translation for 'invalid taxonomy id'.

Invalid taxonomy id was not translated.
Account for current_order returning 'nil' in OrdersController#update

Fixes #2750

Conflicts:
	core/config/locales/en.yml
	core/spec/controllers/spree/orders_controller_spec.rb
Replace stub! with stub.

See David Chelimsky's explanation: http://www.mail-archive.com/rspec-users@rubyforge.org/msg13270.html

Fixes #2898
Respond with a blank 204 status when deleting a taxon

Fixes #2747