Improve error message if interface is missing

[fako1024]

Right now slimcap fires off a quite generic message ("interface does not exist or is unsupported") if either of the two happens. For a user (especially engineers) this might be misleading (as in: one might thing we are missing for support although a configured interface simply doesn't exist). These two should be independent errors. Since we panic for unsupported interfaces (which is intentional so we get people to actually open an issue 😛) we only need to adapt the text for the already existing error.

DoD

• Adapt error message (remove part about "unsupported")