[issue2920] Patch to print symbolic value or errno in EnvironmentError.__str__()

[Terry J. Reedy]

Changes by Terry J. Reedy <tjreedy at udel.edu>:


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versions:  -Python 2.6, Python 2.7, Python 3.1

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue2920>
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