Previously, we would inspect the combination of (return_value, exception_state)
to try to determine what the behavior of future rewrites will be.  For example,
if no attribute was returned and no exception was thrown, then we would assume
that all future times through the rewrite the behavior would be the same.  This
has caused a few bugs, and I'm about to add other cases where it won't be possible
to examine the current function's return value to determine what the future
rewrites will do.

So instead, have getattr-like functions return a "rewriter return convention" flag
that says how future rewrites will behave.