Improvements in proposal controller

`if..else` is better than unless with return.
On failed update render `edit` template and not render `new` template.
On failed save redirect to index path and not render `new` template.
Change `destory` to `withdraw`, cause `destroy` makes DELETE request when
we are updating the event record.
Check for success for save in withdraw (save can fail for reasons other than validation).

config/routes.rb

@@ -95,11 +95,12 @@ Osem::Application.routes.draw do
 
   resources :conference, only: [:index, :show] do
     resource :program, except: :destroy do
-      resources :proposal do
+      resources :proposal, except: :destroy do
         get 'commercials/render_commercial' => 'commercials#render_commercial'
         resources :commercials, only: [:create, :update, :destroy]
         resources :event_attachment, controller: 'event_attachments'
         member do
+          patch '/withdraw' => 'proposal#withdraw'
           patch '/confirm' => 'proposal#confirm'
           patch '/restart' => 'proposal#restart'
         end