More fine grained error message when failing export command

When exporting a image to a file using `--export` and the target filename is a directory, wsl should emit an error indicating that the reason for failure was that a directory, rather than a file was specified. Currently it simply displays `Access is denied.` which leads to investigating permissions issues rather than correcting the parameter.

```
PS D:\wsl> wsl --export 'Ubuntu-18.04' D:\wsl
Access is denied.
PS D:\wsl> wsl --export 'Ubuntu-18.04' D:\wsl\ubuntu.tar
```

Reproduce the failure with `wsl --export 'Ubuntu-18.04' D:\wsl`, then trace the `--export` command's handling of its target path. Compare the directory case with the valid `D:\wsl\ubuntu.tar` case. Done means a directory target produces an error explaining that a directory was specified instead of a file.

Written by the indexing model from the issue text.