Use find instead of find_by_id

Better raise a 404 HTML NotFound exception than any other unexpected error.

spec/controllers/budgets/ballots/lines_controller_spec.rb

@@ -0,0 +1,25 @@
+require "rails_helper"
+
+describe Budgets::Ballot::LinesController do
+
+  describe "#load_budget" do
+
+    it "raises an error if budget slug is not found" do
+      controller.params[:budget_id] = "wrong_budget"
+
+      expect do
+        controller.send(:load_budget)
+      end.to raise_error ActiveRecord::RecordNotFound
+    end
+
+    it "raises an error if budget id is not found" do
+      controller.params[:budget_id] = 0
+
+      expect do
+        controller.send(:load_budget)
+      end.to raise_error ActiveRecord::RecordNotFound
+    end
+
+  end
+
+end