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ARROW-2702: [Python] Change a couple of error types in numpy_to_arrow.cc

I quickly reviewed these errors and it seemed like one error that is hit in the test suite might be better as a ValueError but I'm not sure.

Closes #7336 from wesm/ARROW-2702

Authored-by: Wes McKinney <wesm@apache.org>
Signed-off-by: Wes McKinney <wesm@apache.org>