It's in config/locales/server.en.yml under
omniauth_error: "Sorry, there was an error authorizing your account. Perhaps you did not approve authorization?"
I don't know exactly what would trigger an omniauth error, but it looks to have something to do with app/controllers/users/omniautth_callbacks_controller.rb
flash[:error] = I18n.t("login.omniauth_error")
render layout: 'no_ember'