Warn on the target app for pgbackups:transfer

The current mechanism warns on whatever is currently considered
the app being acted on, which may be the source app. Warning
about the source app is confusing at best. This warns about the
target app, unless the target is a raw URL.
Merge pull request #1161 from deafbybeheading/pgbackups-warn-on-correct-app

Warn on the target app for pgbackups:transfer