Issue #74: VideoSourceFFmpeg constructor uses get_ffmpeg_error_desc to report verbose error messages from FFmpeg wherever applicable
Issue #74: VideoSourceFFmpeg constructor uses get_ffmpeg_error_desc to report verbose error messages from FFmpeg wherever applicable

gift-surg/GIFT-Grab,gift-surg/GIFT-Grab,gift-surg/GIFT-Grab,gift-surg/GIFT-Grab