Raise an exception when returning NULL

Signed-off-by: Alberto Planas <aplanas@suse.com>

Author: aplanas

psutil/arch/linux/users.c

@@ -163,6 +163,7 @@ psutil_users_systemd(PyObject *self, PyObject *args) {
     Py_DECREF(py_retlist);
     if (sessions_list)
         free(sessions_list);
+    PyErr_SetString(PyExc_RuntimeError, "cannot get user information via systemd");
     return NULL;
 }
 
@@ -223,5 +224,6 @@ psutil_users_utmp(PyObject *self, PyObject *args) {
     Py_XDECREF(py_tuple);
     Py_DECREF(py_retlist);
     endutent();
+    PyErr_SetString(PyExc_RuntimeError, "cannot get user information via utmp");
     return NULL;
 }