Respond with 404 instead of 500 when snip part does not exist.

lib/vanilla/renderers/base.rb

@@ -95,7 +95,12 @@ def prepare(snip, part, args, enclosing_snip)
       end
 
       def render_without_including_snips(snip, part=:content)
-        process_text(raw_content(snip, part))
+        if content = raw_content(snip, part)
+          process_text(content)
+        else
+          app.response.status = 404
+          %{Couldn't find part "#{part}" for snip "#{snip.name}"}
+        end
       end
 
       # Handles processing the text of the content.

test/pristine_app/current_snip_test.rb

@@ -54,4 +54,21 @@
       assert_equal 404, page.status_code
     end
   end
+
+  context "when the requested part of a snip is missing" do
+    setup do
+      set_main_template "<layout>{current_snip}</layout>"
+      create_snip :name => "test", :content => "test"
+
+      visit "/test/monkey"
+    end
+
+    should "render an explanatory message" do
+      assert page.has_content?(%{Couldn't find part "monkey" for snip "test"})
+    end
+
+    should "set the response code to 404" do
+      assert_equal 404, page.status_code
+    end
+  end
 end