Return missing page when tag does not exist (#2563)

app/controllers/tags_controller.rb

@@ -4,7 +4,7 @@ class TagsController < ApplicationController
   layout 'public'
 
   def show
-    @tag      = Tag.find_by(name: params[:id].downcase)
+    @tag      = Tag.find_by!(name: params[:id].downcase)
     @statuses = @tag.nil? ? [] : Status.as_tag_timeline(@tag, current_account, params[:local]).paginate_by_max_id(20, params[:max_id])
     @statuses = cache_collection(@statuses, Status)
   end

spec/controllers/tags_controller_spec.rb

@@ -12,5 +12,11 @@ RSpec.describe TagsController, type: :controller do
       get :show, params: { id: 'test' }
       expect(response).to have_http_status(:success)
     end
+
+    it 'returns http missing for non-existent tag' do
+      get :show, params: { id: 'none' }
+
+      expect(response).to have_http_status(:missing)
+    end
   end
 end