vm: Factor out exception block setup to a macro.

Will be reused in WITH bytecodes.
vm: Make sure that exception triple is <type, instance, traceback>.

This reduntant triple is one of the ugliest parts of Python, which they
chickened out to fix in Python3. We really should consider passing just
as single exception instance (without breaking Python-level APIs of course),
but until we do, let's follow CPython layout.