When fetching WAL for a basebackup, report errors with a sensible TLI.

When fetching WAL for a basebackup, report errors with a sensible TLI.

The previous code used ThisTimeLineID, which need not even be
initialized here, although it usually was in practice, because
pg_basebackup issues IDENTIFY_SYSTEM before calling BASE_BACKUP,
and that initializes ThisTimeLineID as a side effect. That's not
really good enough, though, not only because we shoudn't be counting
on side effects like that, but also because the TLI could change
meanwhile. Fortunately, we have convenient access to more meaningful
TLI values, so use those instead.

Because of the way this logic is coded, the consequences of using
a possibly-incorrect TLI here are no worse than a slightly confusing
error message, but it's still worth fixing.