Detach and close watcher explicitly on failure at #start_watchers

Watchers should be detached and closed explicitly due to Cool.io
objects.

Signed-off-by: Hiroshi Hatake <hatake@clear-code.com>

lib/fluent/plugin/in_tail.rb

@@ -418,7 +418,8 @@ def start_watchers(targets_info)
           @tails[target_info] = tw
         rescue Errno::ENOENT
           $log.warn "stat() for #{target_info.path} failed with ENOENT. Drop tail watcher for now."
-          # explicitly detach and close watcher `tw`, or is it being garbage-collected?
+          # explicitly detach and close watcher `tw`.
+          stop_watchers(target_info, immediate: true, unwatched: true)
         end
       }
     end