pg_upgrade: Improve error checking in rewriteVisibilityMap.

pg_upgrade: Improve error checking in rewriteVisibilityMap.

In the old logic, if read() were to return an error, we'd silently stop
rewriting the visibility map at that point in the file.  That's safe,
but reporting the error is better, so do that instead.

Report by Andres Freund.  Patch by Masahiko Sawada, with one correction
by me.