IDL compiler: Fix 2 warts with new exceptionState

There's one spacing typo, and one unnecessary line
(declares an ExceptionState even if no typechecking performed);
this fixes them, and also does some minor refactoring of Perl and Python.

Followup to:
WIP: Migrate generated bindings to new ExceptionState constructor.
https://codereview.chromium.org/99083002/
and:
IDL compiler: sync Python to r163178
https://codereview.chromium.org/105883006/

BUG=239771
BUG=270033

Review URL: https://codereview.chromium.org/103523003

git-svn-id: svn://svn.chromium.org/blink/trunk@163217 bbb929c8-8fbe-4397-9dbb-9b2b20218538