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ARROW-866: [Python] Be robust to PyErr_Fetch returning a null exc value

cc @BryanCutler. This was a tricky one. I am not sure how to reproduce with our current code -- I reverted the patch from ARROW-822 to get a reproduction so I could fix this. Now, the error raised is:

```
/home/wesm/code/arrow/python/pyarrow/_error.pyx in pyarrow._error.check_status (/home/wesm/code/arrow/python/build/temp.linux-x86_64-2.7/_error.cxx:1324)()
     58             raise ArrowInvalid(message)
     59         elif status.IsIOError():
---> 60             raise ArrowIOError(message)
     61         elif status.IsOutOfMemory():
     62             raise ArrowMemoryError(message)

ArrowIOError: IOError: Error message was null
```

I'm not sure why calling `tell` on the socket object results in a bad exception state, but in any case it seems that the result of `PyErr_Fetch` cannot be relied upon to be non-null even when `PyErr_Occurred()` returns non-null

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #606 from wesm/ARROW-866 and squashes the following commits:

fa395cd [Wes McKinney] Enable other kinds of Status errors to be returned
0bd11c2 [Wes McKinney] Consolidate error handling code a bit
9d59dd2 [Wes McKinney] Be robust to PyErr_Fetch returning a null exc value