Correctly throw an exception if PyErr was set

Author: rhaschke

include/pybind11/detail/smart_holder_type_casters.h

@@ -632,7 +632,7 @@ struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_l
         auto src_raw_ptr = src.get();
         auto st          = type_caster_base<T>::src_and_type(src_raw_ptr);
         if (st.first == nullptr)
-            return none().release(); // PyErr was set already.
+            throw error_already_set();
 
         void *src_raw_void_ptr         = static_cast<void *>(src_raw_ptr);
         const detail::type_info *tinfo = st.second;