Fix crash + drop deprecated GtkFileChooser + add Toast overlay to VPN import dialog UI

The VPN import experience would throw a couple of GtkMessageDialogs with error messages. We should be using Toasts instead.

Old screenshot

Notice the unnecessary level of detail of the error message.

The changes in this MR turn some of that detail into g_warning (command line errors) and instead goes with a human-readable summary of the issue. The new sentence NEEDS WORK as it gets ellipsised.

New screenshot

Fix crash