Adjust script to break on instance index missing

lib/tasks/cf.rake

@@ -1,12 +1,8 @@
 namespace :cf do
   desc "Only run on the first application instance"
   task on_first_instance: :environment do
-    instance_index = -1
-    begin
-      instance_index = JSON.parse(ENV["VCAP_APPLICATION"])["instance_index"]
-    rescue StandardError
-      nil
-    end
+    # We expect this information to be always available or break otherwise
+    instance_index = JSON.parse(ENV["VCAP_APPLICATION"])["instance_index"]
     exit(0) unless instance_index.zero?
   end
 end