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.