optional attribute, only clear the exception when the internal getattr
operation raised AttributeError.  Many places in this file already had
that policy; but just as many didn't, and there didn't seem to be any
rhyme or reason to it.  Be consistently cautious.

Question: should I backport this?  On the one hand it's a bugfix.  On
the other hand it's a change in behavior.  Certain forms of buggy or
just weird code would work in the past but raise an exception under
the new rules; e.g. if you define a __getattr__ method that raises a
non-AttributeError exception.