golang, errors, fmt: Error chaining (Python)
Following errors model in Go and fd95c88a (golang, errors, fmt: Error chaining (C++/Pyx)) let's add support at Python-level for errors to wrap each other and to be inspected/unwrapped: - an error can additionally provide way to unwrap itself, if it provides .Unwrap() method. .__cause__ is not taken into account yet, but will be in a follow-up patch; - errors.Is(err) tests whether an item in error's chain matches target; - `fmt.Errorf("... : %w", ... err)` is similar to `"... : %s" % (..., err)` but resulting error, when unwrapped, will return err. - errors.Unwrap is not exposed as chaining through both .Unwrap() and .__cause__ will need more than just "current element" as unwrapping state (i.e. errors.Unwrap API is insufficient - see next patch), and in practice users of errors.Unwrap() are very seldom. Support for error chaining through .__cause__ will follow in the next patch. Top-level documentation is TODO. See https://blog.golang.org/go1.13-errors for error chaining overview.
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