Correctly report Python exceptions set but not thrown during import.

[anntzer]

Consider the following example:

#include <pybind11/pybind11.h>
PYBIND11_MODULE(python_example, m) {
    PyErr_SetString(PyExc_ImportError, "oops");
}

Currently, importing this module will result in

SystemError: initialization of python_example raised unreported exception

because the Python exception is not converted to a C++ one
(py::error_already_set). Fix that by checking the error flag, so that
one instead gets

ImportError: oops

Real use cases include numpy's import_array() or pycairo's
import_cairo(), which can set the exception flag (and return -1) on
failure. Currently the correct way to handle them with pycairo is

if (import_foo() < 0) { m.ptr() = nullptr; return; }

and this patch removes the need for m.ptr() = nullptr;.

No tests for now for the same reason as in #1362 (I think any proper unit test would require creating a new module just for that purpose, and I'm not particularly familiar with the testing machinery used by pybind, so guidance would be welcome here. But "it works for me"...).

[anntzer]

Kindly bumping. To the best of my understanding, the test failures seem spurious.

[wjakob]

Hi @anntzer,

the pybind11 way involves turning such errors into exceptions that are propagated and correctly, and setting PyErr_SetString directly within pybind11 code violates that contract. If you call into other libraries with that behavior, you'll have to check for errors and throw py::error_already_set.

Best,
Wenzel

[anntzer]

Ah, I missed the possibility to just throw py::error_already_set in that case, thanks for pointing that out.