Appends the load paths to error message

When `resolve!` raises an error, it would not display the paths that
were used to look up the assets. If we raise this error we can be even
more helpful and display the `config[:path]` as well. This makes it
easier for users to debug their asset paths.

References #252

lib/sprockets/resolve.rb

@@ -60,6 +60,7 @@ def resolve!(path, options = {})
         end
 
         message << " with type '#{options[:accept]}'" if options[:accept]
+        message << "\nLooked up these paths: #{config[:paths]}"
 
         raise FileNotFound, message
       end

test/test_server.rb

@@ -283,7 +283,7 @@ def app
   test "re-throw JS exceptions in the browser" do
     get "/assets/missing_require.js"
     assert_equal 200, last_response.status
-    assert_equal "throw Error(\"Sprockets::FileNotFound: couldn't find file 'notfound' with type 'application/javascript'\\n  (in #{fixture_path("server/vendor/javascripts/missing_require.js")}:1)\")", last_response.body
+    assert_equal "throw Error(\"Sprockets::FileNotFound: couldn't find file 'notfound' with type 'application/javascript'\\nLooked up these paths: #{@env.paths}\\n  (in #{fixture_path("server/vendor/javascripts/missing_require.js")}:1)\")", last_response.body
   end
 
   test "display CSS exceptions in the browser" do