Improve the single HTMLSelectElement exception message.

Trivial, right? As it turns out, this was completely broken: the code
generator generated code that returned early for custom setters that
failed to set. This means that the exception never actually was thrown,
and of course we didn't have any tests for the behavior. *sigh*

Now we do. And now it works.

BUG=270033

Review URL: https://codereview.chromium.org/82553006

git-svn-id: svn://svn.chromium.org/blink/trunk@162549 bbb929c8-8fbe-4397-9dbb-9b2b20218538