Improve Error messages

- Improve error messages in event schedules, schedules and program controller.
- Two respond_to blocks in the update method of program controller.

app/controllers/admin/schedules_controller.rb

@@ -17,7 +17,7 @@ module Admin
                     notice: 'Schedule was successfully created.'
       else
         redirect_to admin_conference_schedules_path(conference_id: @conference.short_title),
-                    error: 'Could not create schedule'
+                    error: "Could not create schedule. #{@schedule.errors.full_messages.join('. ')}."
       end
     end