Throw a SyntaxError if you try to import from __future__

I guess there's actually a real __future__ module that contains
information in it about the special features you just enabled!
Until this change you would get that information, but not actually
enable any features.  The right behavior is to raise a SyntaxError
for all __future__ directives we don't support (which is all of them).

"import __future__" works if you want that...

I'm not sure that throwing a SyntaxError from the IRGenerator is safe,
since there are things that won't get cleaned up, but I guess
it's not that much worse than if it worked; and this SyntaxError will
typically kill the program anyway.