Bad error report when the lxd instance is already running
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### 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
```shell
n/a
```
### Relevant log output
```shell
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'
```
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