testUsdExportStripNamespaces test TF_RUNTIME_ERROR to Python exception detailed error string lost

[ppt-adsk]

In https://github.com/Autodesk/maya-usd/pull/595 the TF_RUNTIME_ERROR to Python translation tested by testUsdExportStripNamespaces loses the detailed error string. Before this pull request, the detailed error message was preserved. The automated test was weakened to allow the more generic error string, but this should be fixed.