[GUI] Telegraphic error instead of bridging prompt

[ricab]

Describe the bug
On Linux, if the selected bridged network is a plain ethernet card (rather than a bridge) and no bridge "contains" it, the GUI will fail to launch the first bridged instance with:

To Reproduce

1. Delete any existing bridges for your network card (if Multipass created one, you can use nmcli connection delete br-enx1234 br-enx1234-child)
   • you can use nmcli connection show to find the relevant connections
2. Click the cogwheel button in one of the entries in the Catalog
3. Toggle Bridged network
4. Attempt to launch

Expected behavior
A bridging prompt, like multipass launch --bridged does.

Additional info

• OS: Linux
• multipass version: 1.15.0 RC4
• multipass get local.driver: QEMU or LXD

[ricab]

The workaround is to launch once via the CLI: multipass launch --bridged and answer yes to the prompt. Subsequent launches will then work in the GUI too.