Bad error report when the lxd instance is already running

Bug Description

I had a pack command crash for unrelated reasons, which left the lxd instance running. When running craftapp pack again I got the following error:

Launching managed ubuntu 22.04 instance...                                                                                                                                                   
Starting instance                                                                                                                                                                            
Failed to start 'craftapp-hello-on-amd64-for-amd64-1'.                                                                                                                                    
Full execution log: '/home/ubuntu/.local/state/craftapp/log/craftapp-20240703-093523.724743.log'   

To Reproduce

Run craftapp pack, and in another terminal run craftapp pack in the same directory again

part yaml

n/a

Relevant log output

2024-07-03 09:35:29.737 craft_providers.lxd.errors.LXDError: Failed to start 'craftapp-hello-on-amd64-for-amd64-1'.
2024-07-03 09:35:29.737 * Command that failed: 'lxc --project craftapp start local:craftapp-hello-on-amd64-for-amd64-1'
2024-07-03 09:35:29.737 * Command exit code: 1
2024-07-03 09:35:29.737 * Command standard error output: b'Error: The instance is already running\n'

Start at the pack command entry point and trace the LXD instance-start path described in the log. Reproduce the issue by running craftapp pack twice in the same directory, then inspect how the 'instance is already running' error is handled. Done means the second command reports a useful, intentional result instead of the raw failed-start error.