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.