Report errors from incompatible output formats

[keis]

Finding the correct combination of output options is tricky. Gstreamer has no more details but "it failed" AFAICT. Running with GST_DEBUG=3 can help figure out what's going on.

Or document some working incantations

[keis]

Working mp4 for reference is

output {
    container "video/quicktime,variant=iso"
    video "video/x-h264"
    audio "audio/mpeg,mpegversion=1"
}