- if we try guarding after a mutation
- if we use all of our scratch space

Now, just set a "failed" flag internally and which prevents committing.

The motivation for the first part is trying to get rewrite calls to tp_getattro;
if the rewrite is from getattr then it will succeed, but if it comes from
callattr then we will want to do some more guards after the tp_getattro.  We
could try to pass that state around, but for now just use the 'failed' approach.