`containertool` should print better error messages for authentication failures

[euanh]

When authentication fails, containertool prints the 403 error without any further information:
Error: Registry returned an unexpected HTTP error code: 403

[Mrigankkh]

Hello! Can I take this up?

[euanh]

@Mrigankkh Yes, pull requests are very welcome.

I think the first problem is that the HTTPResponse struct we have in the error enum here doesn't include information such as the hostname of the registry.

One approach would be to capture the HTTPRequest in the error as well, and then update the CustomStringConvertible extension here to include the name of the registry and a better textual description of what went wrong. That would allow the user to know that the credentials were rejected, and which registry rejected them.