Distinguish between AcquireExternalFD and epoll_create1 / kqueue failing

The error messages in CreateWaitEventSet() made it hard to know whether the
syscall or AcquireExternalFD() failed. This is particularly relevant because
AcquireExternalFD() imposes a lower limit than what would cause syscalls fail
with EMFILE.

I did not change the message in libpqsrv_connect_prepare(), which is the one
other use of AcquireExternalFD() in our codebase, as the error message already
is less ambiguous.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/xjjx7r4xa7beixuu4qtkdhnwdbchrrpo3gaeb3jsbinvvdiat5@cwjw55mna5of

Author: anarazel

src/backend/storage/ipc/latch.c

@@ -811,10 +811,7 @@ CreateWaitEventSet(ResourceOwner resowner, int nevents)
 
 #if defined(WAIT_USE_EPOLL)
 	if (!AcquireExternalFD())
-	{
-		/* treat this as though epoll_create1 itself returned EMFILE */
-		elog(ERROR, "epoll_create1 failed: %m");
-	}
+		elog(ERROR, "AcquireExternalFD, for epoll_create1, failed: %m");
 	set->epoll_fd = epoll_create1(EPOLL_CLOEXEC);
 	if (set->epoll_fd < 0)
 	{
@@ -823,10 +820,7 @@ CreateWaitEventSet(ResourceOwner resowner, int nevents)
 	}
 #elif defined(WAIT_USE_KQUEUE)
 	if (!AcquireExternalFD())
-	{
-		/* treat this as though kqueue itself returned EMFILE */
-		elog(ERROR, "kqueue failed: %m");
-	}
+		elog(ERROR, "AcquireExternalFD, for kqueue, failed: %m");
 	set->kqueue_fd = kqueue();
 	if (set->kqueue_fd < 0)
 	{