Improve message about failed transaction log archiving

The old phrasing appeared to imply that the failure was terminal.
Improve that by indicating that archiving will be tried again later.

src/backend/postmaster/pgarch.c

@@ -503,7 +503,7 @@ pgarch_ArchiverCopyLoop(void)
 				if (++failures >= NUM_ARCHIVE_RETRIES)
 				{
 					ereport(WARNING,
-							(errmsg("transaction log file \"%s\" could not be archived: too many failures",
+							(errmsg("archiving transaction log file \"%s\" failed too many times, will try again later",
 									xlog)));
 					return;		/* give up archiving for now */
 				}