Use 'raise from' in initialization

Author: virtuald

include/pybind11/detail/common.h

@@ -239,7 +239,8 @@ extern "C" {
 
 #define PYBIND11_CATCH_INIT_EXCEPTIONS \
         catch (pybind11::error_already_set &e) {                               \
-            PyErr_SetString(PyExc_ImportError, e.what());                      \
+            e.caused(PyExc_ImportError, "initialization failed");              \
+            e.restore();                                                       \
             return nullptr;                                                    \
         } catch (const std::exception &e) {                                    \
             PyErr_SetString(PyExc_ImportError, e.what());                      \