gh-122311: Improve and unify pickle errors (GH-122771)

* Raise PicklingError instead of UnicodeEncodeError, ValueError
  and AttributeError in both implementations.
* Chain the original exception to the pickle-specific one as __context__.
* Include the error message of ImportError and some AttributeError in
  the PicklingError error message.
* Unify error messages between Python and C implementations.
* Refer to documented __reduce__ and __newobj__ callables instead of
  internal methods (e.g. save_reduce()) or pickle opcodes (e.g. NEWOBJ).
* Include more details in error messages (what expected, what got).
* Avoid including a potentially long repr of an arbitrary object in
  error messages.

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