Make AuthenticationFailedError consistent with other client errors

[chlowell]

Credentials return AuthenticationFailedError when authentication fails. That type has a RawResponse() method which returns any response the credential received. Naively, it seems AuthenticationFailedError should instead wrap azcore.ResponseError, to enable consistent error handling. Otherwise the API compels applications to handle authentication failures differently than other errors:

err := client.Fail()
var res *http.Response
var authErr azidentity.AuthenticationFailedError
if errors.As(err, &authErr) {
	res = authErr.RawResponse()
}
var respErr azcore.ResponseError
if errors.As(err, &respErr) {
	res = respErr.RawResponse
}

[chlowell]

Action items from offline discussion:

• make AuthenticationFailedError a struct
  • with a RawResponse field
  • Error() should return a string like ResponseError.Error()