Test an "except +" function returning <object>NULL

This provides a test case for #2603

tests/run/cpp_exceptions.pyx

@@ -21,6 +21,8 @@ cdef extern from "cpp_exceptions_helper.h":
     cdef void raise_typeerror() except +
     cdef void raise_underflow() except +
 
+    cdef raise_or_throw(bint py) except +
+
     cdef cppclass Foo:
         int bar_raw "bar"(bint fire) except +
         int bar_value "bar"(bint fire) except +ValueError
@@ -98,6 +100,19 @@ def test_underflow():
     """
     raise_underflow()
 
+def test_func_that_can_raise_or_throw(bint py):
+    """
+    >>> test_func_that_can_raise_or_throw(0)
+    Traceback (most recent call last):
+    ...
+    RuntimeError: oopsie
+    >>> test_func_that_can_raise_or_throw(1)
+    Traceback (most recent call last):
+    ...
+    ValueError: oopsie
+    """
+    raise_or_throw(py)
+
 def test_int_raw(bint fire):
     """
     >>> test_int_raw(False)

tests/run/cpp_exceptions_helper.h

+#include <Python.h>
 #include <ios>
 #include <new>
 #include <stdexcept>
@@ -61,3 +62,11 @@ void raise_typeerror() {
 void raise_underflow() {
     throw std::underflow_error("underflow_error");
 }
+
+PyObject *raise_or_throw(int py) {
+    if (!py) {
+        throw std::runtime_error("oopsie");
+    }
+    PyErr_SetString(PyExc_ValueError, "oopsie");
+    return NULL;
+}