Fail gracefully when file is not found for a document

[burisu]

A TypeError occurred in documents#show:

no implicit conversion of nil into String
app/controllers/backend/documents_controller.rb:51:in `block (2 levels) in show'

---

Request:

• URL : http://demo.ekylibre.farm/backend/documents/7.jpg
• HTTP Method: GET
• Parameters : {"controller"=>"backend/documents", "action"=>"show", "id"=>"7", "format"=>"jpg"}
• Timestamp : 2015-03-11 16:57:16 +0000
• Server : ma001
• Rails root : /var/www/sheep-demo/releases/20150311023911-ekylibre-1.1.0.rc2
• Process: 5542

---

Backtrace:

app/controllers/backend/documents_controller.rb:51:in block (2 levels) in show' app/controllers/backend/documents_controller.rb:46:inshow'