Error handling

[dhardy]

RFD's current API does not allow for error handling. It should.

There is at least one case where rfd fails and I would expect it to fail: when attempting to invoke native file dialogs through xdg-portal from a container which lacks appropriate support (#242).

What I would expect to happen:

• if both xdg-portal and gtk3 features are enabled, transparently fall back to gtk3
• if not, report an error; this allows falling back e.g. to a toolkit-native file dialog (e.g. https://crates.io/crates/egui-file-dialog)

Note that in this situation (at least some) KDE / Gnome / other apps will attempt to use the xdg-portal dialog if available, falling back to a native dialog if not.