[enhancement]: Don't delete failed downloads

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Contact Details

_No response_

### What should this feature add?

When a model fails to install or isn't recognized (due to an unsupported model format), Invoke should keep the downloaded files in a directory instead of deleting them.

### Alternatives

_No response_

### Additional Content

I tried downloading FLUX.1 [dev] using Invoke. It downloaded the model but failed to install it.


```
[2025-03-16 22:15:08,695]::[ModelInstallService]::INFO --> Model download complete: black-forest-labs/FLUX.1-dev
[2025-03-16 22:15:08,702]::[ModelInstallService]::INFO --> Model install started: black-forest-labs/FLUX.1-dev
[2025-03-16 22:15:08,744]::[ModelInstallService]::ERROR --> Model install error: black-forest-labs/FLUX.1-dev
InvalidModelConfigException: Unknown base model for G:\03 - AI\Image Generation\InvokeAI\models\tmpinstall_az0bcd50\FLUX.1-dev
```
I thought that Invoke didn't support FLUX, so I went into the models folder to transfer the files to ComfyUI, but Invoke had already deleted the downloaded files.
Because of that, I waited several hours for nothing, and now I need to download 21GB of files again.