Improve cred store message for MacOS

### Contact Details

dmitry@spikhalskiy.com

### What happened?

If bk is configured with `smh` cred store on MacOS, the error message is not helpful to identify the rootcase:
`Error: OAuth login requires an available credential store to persist your access token and refresh token`.
Please consider improving the message with more details like "Configured keystore: `smh`. `keyring` or `auto` is recommended on MacOS"

### Version

3.56.0

### What environment are you seeing the problem on?

Local Development

### Relevant log output

```shell
Waiting for authentication...
Exchanging authorization code for token...
Error: OAuth login requires an available credential store to persist your access token and refresh token
```

Start by locating the OAuth login error message and the credential-store configuration handling in the Buildkite CLI. Reproduce the MacOS case with `smh`, then verify that the error identifies the configured store and recommends `keyring` or `auto` when appropriate.

Written by the indexing model from the issue text.