Tests for PreconditionFailed error messages.

Change to relation.rb to bring element selection in-line with that of way, so to return nil to be caught later, rather than raise an error.
Change to way.rb to clarify which way on diff upload.

test/functional/changeset_controller_test.rb

@@ -397,6 +397,7 @@ EOF
     post :upload, :id => 2
     assert_response :precondition_failed, 
       "shouldn't be able to upload a invalid deletion diff: #{@response.body}"
+    assert_equal "Precondition failed: Way 3 still used by relation 1.", @response.body
 
     # check that nothing was, in fact, deleted
     assert_equal true, Node.find(current_nodes(:node_used_by_relationship).id).visible