Don't show odd errors when using remote URIs

[sergio-costas]

When a remote URI is selected in the portal, and the application is inside a flatpak or a snap, it is not possible to get a local path.

This MR detects this and shows an specific error, instead of the odd errors that previously were shown.

[sergio-costas]

This patch is the bare minimum for https://github.com/flatpak/xdg-desktop-portal/issues/820 and https://github.com/flatpak/xdg-desktop-portal/issues/213 . Ideally, apply https://github.com/flatpak/xdg-desktop-portal/pull/822 instead.