Emit log when restore_command succeeds but archived file faills to be restored.

Previously, when restore_command claimed to succeed but failed to restore
the file with the right name, for example, due to mis-configuration of
restore_command, no log message was reported. Then the recovery failed
later with an error message not directly related to the issue.

This commit changes the recovery so that a log message is emitted in
this error case. This would enable us to investigate what happened in
this case more easily.

Author: Jeff Janes, Fujii Masao
Reviewed-by: Pavel Borisov, Kyotaro Horiguchi
Discussion: https://postgr.es/m/CAMkU=1xkFs3Omp4JR4wMYWdam_KLuj6LXnTYfU8u3T0h=PLLMQ@mail.gmail.com

src/backend/access/transam/xlogarchive.c

@@ -220,11 +220,12 @@ RestoreArchivedFile(char *path, const char *xlogfname,
 		else
 		{
 			/* stat failed */
-			if (errno != ENOENT)
-				ereport(FATAL,
-						(errcode_for_file_access(),
-						 errmsg("could not stat file \"%s\": %m",
-								xlogpath)));
+			int			elevel = (errno == ENOENT) ? LOG : FATAL;
+
+			ereport(elevel,
+					(errcode_for_file_access(),
+					 errmsg("could not stat file \"%s\": %m", xlogpath),
+					 errdetail("restore_command returned a zero exit status, but stat() failed.")));
 		}
 	}