ARROW-866: [Python] Normalize PyErr exc_value to be more predictable
ARROW-866: [Python] Normalize PyErr exc_value to be more predictable

It is possible when using `PyErr_Fetch(&exc_type, &exc_value, &traceback)` for the `exc_value` to be a string, tuple or NULL.  Calling `PyErr_Normalize` after this will cause `exc_value` to always be a valid object of the same type as `exc_type` which can then be converted to a string predictably.

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #630 from BryanCutler/python-pyerr_normalize-ARROW866 and squashes the following commits:

fb93356 [Bryan Cutler] use PyObjectStringify to be Unicode safe
d56c6bf [Bryan Cutler] Added PyErr_NormalizeException to CheckPyError to make more predictable exc_value

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