fix: print pack and lock errors on stderr

The lock refusal, "no pack for", "no manifest for" and "no pack to
verify" went to stdout, so a run redirected to a log exited 1 with
nothing on the terminal. A test reads both scripts and refuses an ERROR
line printed to stdout.

tests/test_artifact_lock.py

@@ -108,7 +108,7 @@ class PackLockCliTest(unittest.TestCase):
                 ]
             )
         self.assertEqual(proc.returncode, 1, proc.stdout + proc.stderr)
-        self.assertIn("is in use by another run", proc.stdout)
+        self.assertIn("is in use by another run", proc.stderr)
 
     def test_every_writing_mode_refuses_a_locked_output(self):
         """Two of seven modes took the lock, and five wrote straight into it.
@@ -153,7 +153,7 @@ class PackLockCliTest(unittest.TestCase):
                 ]
             )
         self.assertEqual(proc.returncode, 1, proc.stdout + proc.stderr)
-        self.assertIn("is in use by another run", proc.stdout)
+        self.assertIn("is in use by another run", proc.stderr)
 
     def test_pipeline_refuses_locked_output(self):
         with artifact_lock(self.dir):
@@ -167,7 +167,7 @@ class PackLockCliTest(unittest.TestCase):
                 ]
             )
         self.assertEqual(proc.returncode, 1, proc.stdout + proc.stderr)
-        self.assertIn("is in use by another run", proc.stdout)
+        self.assertIn("is in use by another run", proc.stderr)
 
 
 class HeldLockTest(unittest.TestCase):