Custom errors for HasClient, HasServer

[gdeest]

This PR applies the approach from #1289 to HasServer and HasClient.

The context parameter is missing from the missing instance message in one case, but I am not sure it is a big enough problem to warrant making the code more complex.

In the process, I realized there was already a similar mechanism in place to detect usage errors for HasServer. The mechanism #1289 supersedes it, as it produces more informative messages, displays the number of missing parameters, etc.

[gdeest]

OK, I consider the issue of the missing context parameter relatively benign: we are not pattern-matching on the context, but on the API ; the context is irrelevant to the error message, so I'd rather avoid making the error generation machinery more complex for little reason.

Moreover, since we are talking about type errors, we can decide to change how these are generated at any point without consequence on our users, so I am going to merge.