Propagate XLogFindNextRecord error to callers

Hi,

Currently, XLogFindNextRecord errormsg is ignored and callers will only
output a generic 'could not find a valid record' message without details.
Additionally, invalid page header won't go through XLogReadRecord, leaving
the error in state->errormsg_buf.

This patch propagates XLogFindNextRecord's error message to the callers and
displays it. In case of an invalid page header, the errormsg is filled with
errormsg_buf content.

With this patch, pg_waldump will now have the following output when reading
a file with an invalid header:
pg_waldump: error: could not find a valid record after D80/5C000000:
invalid magic number D116 in WAL segment 0000001400000D8000000017, LSN
D80/5C000000, offset 0

Regards,
Anthonin Bonnefoy