Non-HTTP errors previously resulted in a panic due to dereferencing the
resp pointer while it was nil, as part of rendering the error message.
This commit changes the error message formatting to cope with a nil
response, and extends test coverage.
Fixes#24384
By adding this method you now only have to pass a `*disco.Disco` object around in order to do discovery and use any configured credentials for the discovered hosts.
Of course you can also still pass around both a `*disco.Disco` and a `auth.CredentialsSource` object if there is a need or a reason for that!
Alisdair McDiarmid