bug: ardupilot-manager: failed firmware fetch does not update version selector properly

[ES-Alexander]

Bug description

If firmware fails to fetch for a new vehicle type then there should not be previous firmwares still available from a previous vehicle type - the version dropdown should revert to "No Data Available".

Steps to reproduce

1. Fetch firmwares for a vehicle type
2. Prevent new firmwares from fetching (e.g. turn off wifi)
3. Try to fetch firmwares for a new vehicle type
4. Fetch fails, but the old firmware versions are still available in the dropdown

Primary pain point(s)

Can result in installing the wrong firmware without realising.

This came up while I was testing #1153 and I found if I switched to Copter the fetch would fail but the firmwares from the previous fetch would still be there, so I could click install on a version and end up with e.g. Plane firmware when I expected Copter.

Prerequisites

• [X] I have checked to make sure that a similar request has not already been filed or fixed.