auth error for non existing directory

If you are looking at a harbor registry for a particular path that doesn't exist, the error can be miss-leading:

```sh
./seaglass repos private-registry.venafi.cloud/public --recursive
Error: listing repositories: listing repositories: [GET /projects/{project_name}/repositories][403] listRepositoriesForbidden &{Errors:[0x140003a2100]}
Usage:
seaglass repos [flags]

Flags:
-h, --help help for repos
--recursive List repositories recursively
```

I only came across this as I was comparing our Venafi public registry with the private one:

```sh
./seaglass repos registry.venafi.cloud/public --recursive
```

This could happen regularly:
- Because it's often hard to know what the path should be
- A copy paste fail from some doc site, missing the last character for example

I'll take a guess that an OCI repo returns 403 if your reference a path that doesn't exist.
But if there was some way of validating the path doesn't exist and feeding that back in the error, it would be a nicer experience for a user.

Eg. maybe something like:

```sh
./seaglass repos private-registry.venafi.cloud/public --recursive

Error: Path does not exist: private-registry.venafi.cloud/public

Usage:
seaglass repos [flags]

Flags:
-h, --help help for repos
--recursive List repositories recursively
```