Feedback of missing miner binaries is missing in GUI

[krichter722]

adding a custom miner (in my case all three types CUDA, CG and Ufasoft) causes nothing, but a message in the form of

OSError: [Errno 2] Datei oder Verzeichnis nicht gefunden: '/mnt/DATA/sources/poclbm/miners/puddinpop'

("Datei oder Verzeichnis nicht gefunden" means "No such file or directory" - LANG is ignored) to be logged. This could already be more verbose, because these are the default value and there has to be a reason why they are set. And there's no feedback in the GUI at all. The fact that the python frontend is apparently trying to execute/executing .exe file isn't very intuitive neither and needs explanation in the application feedback (logs and GUI).

experienced with 5c6b314