IDL compiler: Exceptions

Exceptions are basically very simple interfaces, and compiled almost
identically (only one change to generated code).
Thus just add an |is_exception| flag, and fill in stub values for
expected fields.

Also improve test case.
There's only one exception (DOMException.idl), and this gives full coverage.

Docs:
http://www.w3.org/TR/WebIDL/#idl-exceptions

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/110643008

git-svn-id: svn://svn.chromium.org/blink/trunk@164084 bbb929c8-8fbe-4397-9dbb-9b2b20218538