IDL compiler: [RaisesException] for non-void methods

Other case, caught while checking test cases.
(Most [RaisesException] methods are void, but not all.)

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/137593005

git-svn-id: svn://svn.chromium.org/blink/trunk@165022 bbb929c8-8fbe-4397-9dbb-9b2b20218538