IDL compiler: simplify ExceptionState code in overloads

Slightly shorter by putting variable declaration+initialization at use,
rather than at top. This removes a extra branch and some complexity.

Followup to:
Generate a bit less code to handle failed arity checks.
https://codereview.chromium.org/105693002/
IDL compiler: sync Python to r163527
https://codereview.chromium.org/111793006/

BUG=270033

Review URL: https://codereview.chromium.org/110893005

git-svn-id: svn://svn.chromium.org/blink/trunk@163670 bbb929c8-8fbe-4397-9dbb-9b2b20218538