Fix UPGRADING.md

Change deprecation msg

UPGRADING.md

@@ -4,7 +4,10 @@ Upgrading Grape
 ### Upgrading to >= 2.1.0
 
 #### Changes in rescue_from
-`rack_response` has been deprecated and `error_response` has been removed. Use `error!` instead.
+
+The `rack_response` method has been deprecated and the `error_response` method has been removed. Use `error!` instead.
+
+See [#2414](https://github.com/ruby-grape/grape/pull/2414) for more information.
 
 #### Grape::Router::Route.route_xxx methods have been removed
 

lib/grape/dsl/inside_route.rb

@@ -180,7 +180,7 @@ def error!(message, status = nil, additional_headers = nil)
       # A Rack::Response object containing the specified message, status, and headers.
       #
       def rack_response(message, status = 200, headers = { Rack::CONTENT_TYPE => content_type })
-        Grape.deprecator.warn('Use error! instead of rack_response')
+        Grape.deprecator.warn('The rack_response method has been deprecated, use error! instead.')
         message = Rack::Utils.escape_html(message) if headers[Rack::CONTENT_TYPE] == 'text/html'
         Rack::Response.new(Array.wrap(message), Rack::Utils.status_code(status), headers)
       end

spec/grape/api_spec.rb

@@ -4437,8 +4437,9 @@ def uniqe_id_route
     end
 
     it 'raises a deprecation' do
-      expect(Grape.deprecator).to receive(:warn).with('Use error! instead of rack_response')
+      expect(Grape.deprecator).to receive(:warn).with('The rack_response method has been deprecated, use error! instead.')
       get 'test'
+      expect(last_response.body).to eq('deprecated')
     end
   end
 end