Previously,
[`SetProperty`](https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setproperty)
implemented step 3 before calling into the inner `set_property` method,
which implements step 1. Therefore if step 3 returned then step 1 never
runs, and can't throw an exception.
Testing: A new web platform test starts to pass
---------
Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>