log a debug message if a capture backend is generally available but isn't capabable of either capture by index or capture by filename #22699
Description
Descripe the feature and motivation
I am working on a few issues pertaining hw accelerated videoio and I experimented with the different backends and settings to get my application to capture from v4l2 while using vaapi for decoding among a few other things.
Anyway it took me considerable time to realize that e.g. the ffmpeg backend doesn't support selecting the capture device by index.
I would like to have at least debug message in place to alert the user to as why capturing failed.
Additional context
VideoCapture cap(0, CAP_FFMPEG);The above line fails without a hint on why.