minimalist exception hierarchy generator

 The failures module lets you define an exception hierarchy quickly and simply.
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 The design goals were:
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  • minimalist interface
  • 80% of features in 20% of lines of code
  • depend only on core modules (nearly achieved)
  • support hierarchical error types
  • identify errors types by name (class) not by parsing strings
  • leave (possibly expensive) trace decisions to the thrower