log fatal errors that we catch

In case we rescue from a fatal error, we want the error and the backtrace to
the error logged, so we can debug later on. This change injects the configured
logger from the rails app to the grape API and logs error as well as backtrace
in a rails-like fashion.

config/routes.rb

@@ -8,6 +8,7 @@ Gitlab::Application.routes.draw do
 
   # API
   require 'api'
+  Gitlab::API.logger Rails.logger
   mount Gitlab::API => '/api'
 
   constraint = lambda { |request| request.env["warden"].authenticate? and request.env['warden'].user.admin? }