fix(console): a serial console worker's failure reaches socketiod's log

- The worker told why a console did not open or closed only in the user's
  terminal (as users see errors: a host that cannot be reached is not
  named). It now also logs the full reason to socketiod's own stderr, the
  journal or docker logs, which socketiod hands it as a descriptor.
- A refused token says the link has expired or the user may no longer
  open the console: a valid link for a user who lost access was told only
  that it had expired.
- A send that takes nothing (0 bytes) waits like a busy stream instead of
  spinning.

console/tests/test_socketiod.py

@@ -242,8 +242,8 @@ class WorkerIsolationTestCase(Base):
         while b"]" not in output:
             select.select([fd], [], [], 5)
             output += os.read(fd, 1024)
-        # 0-2: its terminal, 3: the listing itself
-        self.assertEqual(output.decode().strip(), "['0', '1', '2', '3']")
+        # 0-2: its terminal, 3: socketiod's log, 4: the listing itself
+        self.assertEqual(output.decode().strip(), "['0', '1', '2', '3', '4']")
 
 
 def _open(fd):