The check to test whether the subscription workers were restarting after a
change in the subscription was failing. The reason was that the test was
assuming the walsender started before it reaches the 'streaming' state and
the walsender was exiting due to an error before that. Now, the walsender
was erroring out before reaching the 'streaming' state because it tries to
acquire the slot before the previous walsender has exited.

In passing, improve the die messages so that it is easier to investigate
the failures in the future if any.

Reported-by: Michael Paquier, as per buildfarm
Author: Ajin Cherian
Reviewed-by: Masahiko Sawada, Amit Kapila
Backpatch-through: 10, where this test was introduced
Discussion: https://postgr.es/m/YRnhFxa9bo73wfpV@paquier.xyz