API: on error, include backtrace in json response.

Incidentally fixes issue with error responses inside /sources.json not
including the usual `{"success": false}`.

app/controllers/sources_controller.rb

@@ -9,14 +9,4 @@ class SourcesController < ApplicationController
       format.xml { render xml: @source.to_h.to_xml(root: "source") }
     end
   end
-
-private
-
-  def rescue_exception(exception)
-    respond_with do |format|
-      format.json do
-        render :json => {:message => exception.to_s, :backtrace => exception.backtrace}, :status => :error
-      end
-    end
-  end
 end