Deduplicate NotFound error in client

[pohlm01]

Currently, there are two error types in the Client implementation that essentially have the same meaning: Error::ObjectNotFound and Error::Problem(openadr_wire::problem::Problem) with a 404 error code.

The first is created in the get_*_by_name() methods, if the returned list is empty, while the second gets sent by the VTN, e.g., server.

It would be nice to streamline them somehow, such that a user of the library does not get surprised by at 'NotFound' error, although it properly handled one of the two error types.
I don't have a perfect proposal on how to deal with that ready yet. I could imagine integrating a special handling for a 404 status in the ClientRef::request() method, but there might be better alternatives.

The Error::is_not_found() method should be adopted accordingly.