Add a `client` attribute to `ApiClientError`

[llucax]

What's needed?

The ApiClientError will be used to report errors for many different types of clients, so we need an easy way to map an error to a particular client.

The error already saves the server_url, which could be potentially used for that, but if for some reason any user wants to instantiate 2 clients with the same URL, this won't be enough.

Proposed solution

Save a client property in the error instance too, pointing to the API client that generated the error.