[issue9306] distutils: raise informative error message when cmd_class is None

[Éric Araujo]

Changes by Éric Araujo <merwok at netwok.org>:


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nosy: +merwok
versions:  -Python 2.7, Python 3.3

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue9306>
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