Report detailed errors from XLogFindNextRecord() failures.

Report detailed errors from XLogFindNextRecord() failures.

Previously, XLogFindNextRecord() did not return detailed error information
when it failed to find a valid WAL record. As a result, callers such as
the WAL summarizer, pg_waldump, and pg_walinspect could only report generic
errors (e.g., "could not find a valid record after ..."), making
troubleshooting difficult.

This commit fix the issue by extending XLogFindNextRecord() to return
detailed error information on failure, and updating its callers to include
those details in their error messages.

For example, when pg_waldump is run on a WAL file with an invalid magic number,
it now reports not only the generic error but also the specific cause
(e.g., "invalid magic number").

Author: Anthonin Bonnefoy <anthonin.bonnefoy@datadoghq.com>
Reviewed-by: Mircea Cadariu <cadariu.mircea@gmail.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAO6_XqoxJXddcT4wkd9Xd+cD6Sz-fyspRGuV4Bq-wbXG4pVNzA@mail.gmail.com