cli: Provide next try time on network error

Just like other places in caucase-updater, print the next time of wake up.
This information is useful in logs, for debugging caucase-updater problems.

/reviewed-on nexedi/caucase!6

caucase/cli.py

@@ -74,14 +74,16 @@ class RetryingCaucaseClient(CaucaseClient):
         connection.close() # Resets HTTPConnection state machine.
         # Note: repr(str(exception)) is nicer than repr(exception), without
         # letting non-printable characters through.
+        next_try = datetime.datetime.utcnow() + datetime.timedelta(0, 10)
         print(
-          'Got a network error, retrying in a bit: %s: %r' % (
+          'Got a network error, retrying at %s, %s: %r' % (
+            next_try.strftime(b'%Y-%m-%d %H:%M:%S +0000'),
             exception.__class__.__name__,
             str(exception),
           ),
           file=self._log_file,
         )
-        self._until(datetime.datetime.utcnow() + datetime.timedelta(0, 10))
+        self._until(next_try)
 
 _cryptography_backend = default_backend()