prune: Show device missing dialog before prune dialogs

Now the missing device dialog will be shown before the prune dialog is opened for removable drives. Mounting gvfs repos will still show the spinner immediately.

Also fixes a bug where repositories on removable drives wouldn't get removed from the repo list in the setup dialog.

Closes #163 (closed)

Checklist

• Add entry to the CHANGELOG.md for non-trivial changes