Fix print out for callback error

Yury Matveevrequested to merge
683-callback-error-ins-not-printed into develop

Closes #683 (closed)

The origin is that, compared to the boost, pybind11 fetches error from stack, so the consequent CPython PyErr_Print call just does not have a error in stack to be printed.

Fixed

Edited by Yury Matveev

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