env: Make the env save message a bit more explicit

Since we'll soon have support for multiple environments, the environment
saving message might end up being printed multiple times if the higher
priority environment cannot be used.

That might confuse the user, so let's make it explicit if the operation
failed or not.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>