bigfile/py: Teach storeblk() how to correctly propagate traceback on error
Previously we were limited to printing traceback starting down from just storeblk() via explicit PyErr_PrintEx() - because pybuf was attached to memory which could go away right after return from C function - so we had to destroy that object for sure, not letting any traceback to hold a reference to it. This turned out to be too limiting and not showing full context where errors happen. So do the following trick: before returning, reattach pybuf to empty region at NULL, and this way we don't need to worry about pybuf pointing to memory which can go away -> thus instead of printing exception locally - just return it the usual way it is done with C api in Python. NOTE In contrast to PyMemoryViewObject, PyBufferObject definition is not public, so to support Python2 - had to copy its definition to PY2 compat header. NOTE2 loadblk() is not touched - the loading is done from sighandler context, which simulates as if it work in separate python thread, so it is leaved as is for now.
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