Error text could be more descriptive in KeframeSelector

[amoliski]

I forgot to set the output format for the keyframe selection node in Meshroom- after processing the images, I got an error:
The keyframes selected from the input video have not been written on disk. The keyframes' SfMData file cannot be written.

I had to search through the source to find the reason the file could not be written at line ~690 in /src/aliceVision/keyframe/KeyframeSelector.cpp

// If the current media is a video and there is no output keyframe, the corresponding SfMData file will not be written if (feed.isVideo() && outputExtension == "none") { ALICEVISION_THROW(std::invalid_argument, "The keyframes ...")

It would be really nice if the error included the missing filetype as the reason, and it would be even nicer if extension was checked before the keyframe selection started so the processing isn't wasted.