Improve error message if the command is not decodable

Lib/test/test_sys.py

@@ -509,7 +509,10 @@ class SysModuleTest(unittest.TestCase):
         p = subprocess.Popen([sys.executable, "-c", code], stderr=subprocess.PIPE)
         stdout, stderr = p.communicate()
         self.assertEqual(p.returncode, 1)
-        self.assertIn(br"UnicodeEncodeError:", stderr)
+        lines = stderr.splitlines()
+        self.assertGreaterEqual(len(lines), 2)
+        self.assertEqual(b"Unable to decode the command from the command line:", lines[0])
+        self.assertIn(br"UnicodeEncodeError:", lines[1])
 
     def test_sys_flags(self):
         self.assertTrue(sys.flags)

Modules/main.c

@@ -275,6 +275,7 @@ run_command(wchar_t *command, PyCompilerFlags *cf)
     return ret != 0;
 
 error:
+    PySys_WriteStderr("Unable to decode the command from the command line:\n");
     PyErr_Print();
     return 1;
 }