IDL compiler: treat exceptions as a type of interface

Now that we've implemented exceptions in Python:
The compiler can be significantly simplified by treating exceptions as
a type of interface.

There's only one exception (DOMException), and no new ones are expected;
the only difference in the CG between interfaces and exceptions is one value,
so we want to minimize overhead in supporting exceptions.
This CL minimizes the code: all we need is one added flag ('is_exception').

R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/113193009

git-svn-id: svn://svn.chromium.org/blink/trunk@164223 bbb929c8-8fbe-4397-9dbb-9b2b20218538