bpo-39884: Add method name in "bad call flags" error (GH-18944)

PyDescr_NewMethod() and PyCFunction_NewEx() now include the method
name in the SystemError "bad call flags" error message to ease debug.

Misc/NEWS.d/next/C API/2020-03-12-00-27-26.bpo-39884.CGOJBO.rst

@@ -0,0 +1,2 @@
+:c:func:`PyDescr_NewMethod` and :c:func:`PyCFunction_NewEx` now include the
+method name in the SystemError "bad call flags" error message to ease debug.

Objects/descrobject.c

@@ -888,7 +888,8 @@ PyDescr_NewMethod(PyTypeObject *type, PyMethodDef *method)
             vectorcall = method_vectorcall_O;
             break;
         default:
-            PyErr_SetString(PyExc_SystemError, "bad call flags");
+            PyErr_Format(PyExc_SystemError,
+                         "%s() method: bad call flags", method->ml_name);
             return NULL;
     }
 

Objects/methodobject.c

@@ -56,7 +56,8 @@ PyCFunction_NewEx(PyMethodDef *ml, PyObject *self, PyObject *module)
             vectorcall = cfunction_vectorcall_O;
             break;
         default:
-            PyErr_SetString(PyExc_SystemError, "bad call flags");
+            PyErr_Format(PyExc_SystemError,
+                         "%s() method: bad call flags", ml->ml_name);
             return NULL;
     }