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Am going to close this one because I don't see any straight-forward way around it and because it's technically not a bug (just an undesirable design artifact). The use of TypeError for objects that don't define __len__ is deeply ingrained into Python (20 years). If something inside a __len__ method raises a TypeError, it would be challenging to distinguish that from a missing __len__ method. 
[terry]
It might be possible to create internal subclasses of TypeError, AttributeError and whatnot, but that is a deep change with unknown implications for users, for other python implementations, for performance, for third-party modules, etc. It would need a PEP and is likely to get rejected on a cost-benefit basis -- the substantial API churn wouldn't be worth the microscopic benefit (i.e. most people never encounter this or care about it).
[benjamin]
The __length_hint__ protocol is a public API, so anyone can use it. Also, the issue is a broader than __length_hint__, it is really distinguishing multiple possible meanings for a TypeError raised by a call to __len__.