[issue28214] Improve exception reporting for problematic __set_name__ attributes

[Nick Coghlan]

Changes by Nick Coghlan <ncoghlan at gmail.com>:


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nosy: +Martin Teichmann, Martin.Teichmann
stage:  -> needs patch

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28214>
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