improve error message when registry credential config is not valid

[qweeah]

E.g. pulling an artifact and the target credential cannot be found in the config, current output will be like:

$ oras pull localhost:7000/repo:tag --registry-config auth.config
Error: failed to resolve tag: GET "http://localhost:7000/v2/repo/manifests/tag": credential required for basic auth

The error message is not helpful since

1. It's purely a local error but contains details like HTTP method and URI
2. Doesn't mention how to fix the issue

Suggest to change the error to sth like:

$ oras pull localhost:7000/repo:tag --registry-config auth.config
Error: failed to authenticate when attempting to pull: no valid credential found in auth.config
Please check whether the registry credential stored in the authentication file is correct