Add logging for internal download droplet url endpoint

Co-authored-by: Mona Mohebbi <mmohebbi@pivotal.io>
Co-authored-by: Reid Mitchell <rmitchell@pivotal.io>

Author: monamohebbi

lib/cloud_controller/blobstore/url_generator/local_url_generator.rb

@@ -33,11 +33,24 @@ def admin_buildpack_download_url(buildpack)
       end
 
       def droplet_download_url(droplet)
-        return nil unless droplet
-        return nil unless @droplet_blobstore.exists?(droplet.blobstore_key)
+        unless droplet
+          logger.info('Droplet to be downloaded is nil')
+          return nil
+        end
+
+        unless @droplet_blobstore.exists?(droplet.blobstore_key)
+          logger.info('Blobstore key for droplet to be downloaded not found in blobstore')
+          return nil
+        end
 
         http_basic_auth_uri("/staging/v3/droplets/#{droplet.guid}/download")
       end
+
+      private
+
+      def logger
+        @logger ||= Steno.logger('cc.blobstore.local_url_generator')
+      end
     end
   end
 end