golang: Expose error at Py level
The first step to expose errors and error chaining to Python: - Add pyerror that wraps a pyx/nogil C-level error and is exposed as golang.error at py level. - py errors must be compared by ==, not by "is" - Add (py) errors.New to create a new error from text. - a C-level error that has .Unwrap, is exposed with .Unwrap at py level, but full py-level chaining will be implemented in a follow-up patch. - py error does not support inheritance yet. Top-level documentation is TODO.
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