Improve error messages for some callers of XLogReadRecord()

A couple of code paths related to logical decoding (WAL sender, slot
advancing, etc.) use XLogReadRecord(), feeding on error messages
generated by walreader.c on a failure.  All those messages have no
context, making it harder to spot from where an error could come even if
these should not happen.  All the other callers of XLogReadRecord() do
that already.

Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/YYnTH6OyOwQcAdkw@paquier.xyz

src/backend/replication/logical/logical.c

@@ -605,9 +605,9 @@ DecodingContextFindStartpoint(LogicalDecodingContext *ctx)
 		/* the read_page callback waits for new WAL */
 		record = XLogReadRecord(ctx->reader, &err);
 		if (err)
-			elog(ERROR, "%s", err);
+			elog(ERROR, "could not find logical decoding starting point: %s", err);
 		if (!record)
-			elog(ERROR, "no record found"); /* shouldn't happen */
+			elog(ERROR, "could not find logical decoding starting point");
 
 		LogicalDecodingProcessRecord(ctx, ctx->reader);
 

src/backend/replication/logical/logicalfuncs.c

@@ -276,7 +276,7 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 
 			record = XLogReadRecord(ctx->reader, &errm);
 			if (errm)
-				elog(ERROR, "%s", errm);
+				elog(ERROR, "could not find record for logical decoding: %s", errm);
 
 			/*
 			 * The {begin_txn,change,commit_txn}_wrapper callbacks above will

src/backend/replication/slotfuncs.c

@@ -529,7 +529,8 @@ pg_logical_replication_slot_advance(XLogRecPtr moveto)
 			 */
 			record = XLogReadRecord(ctx->reader, &errm);
 			if (errm)
-				elog(ERROR, "%s", errm);
+				elog(ERROR, "could not find record while advancing replication slot: %s",
+					 errm);
 
 			/*
 			 * Process the record.  Storage-level changes are ignored in

src/backend/replication/walsender.c

@@ -2979,7 +2979,8 @@ XLogSendLogical(void)
 
 	/* xlog record was invalid */
 	if (errm != NULL)
-		elog(ERROR, "%s", errm);
+		elog(ERROR, "could not find record while sending logically-decoded data: %s",
+			 errm);
 
 	if (record != NULL)
 	{