Emit a LOG message when the postmaster stops because of a failure in
the startup process.  There already is a similar message if we exit
for that reason during PM_STARTUP phase, so it seems inconsistent
that there was none if the startup process fails later on.

Also emit a LOG message when the postmaster stops after a crash
because restart_after_crash is disabled.  This seems potentially
helpful in case DBAs (or developers) forget that that's set.
Also, it was the only remaining place where the postmaster would
do an abnormal exit without any comment as to why.

In passing, remove an unreachable call of ExitPostmaster(0).

Discussion: https://postgr.es/m/194914.1621641288@sss.pgh.pa.us