Exception Handling on Feed#fetch_favicon!

To prevent from raising 500 error by the following exception.
favicon 取得時に以下の例外で500が発生するのを防ぎます。

```
MiniMagick::Error (Command ("identify -quiet -ping /var/folders/rf/b4gsb4xs62s88_072yd95fgm0000gp/T/mini_magick20130320-87213-a29nqg.ico") failed: {:status_code=>1, :output=>"identify: improper image header `/va
r/folders/rf/b4gsb4xs62s88_072yd95fgm0000gp/T/mini_magick20130320-87213-a29nqg.ico' @ error/icon.c/ReadICONImage/307.\n"}):
  app/models/feed.rb:115:in `block in fetch_favicon!'
  app/models/feed.rb:104:in `each'
  app/models/feed.rb:104:in `fetch_favicon!'
  app/controllers/api/feed_controller.rb:203:in `fetch_favicon'
```