Silent option handling can lead to pain

Hi,

I just spent an hour or so debugging a painful, silly user error.

The problem was that I commented out some lines in /etc/libao.conf by prepending #.

Some drivers, like alsa, allow options with unknown keys to be set. Other drivers, like pulse, do not.

I had a commented out line with the alsa driver (which worked), and could not figure out why the pulse driver would mysteriously fail!

A quick fix to help stupid people like me would be to add a verbose error message in audio_out.c:

  if (!device->funcs->set_option(device, options->key, options->value)) {
    /* Problem setting options */
    aerror("Unable to set option %s\n", options->key);
    return AO_EOPENDEVICE;
  }

It probably also makes sense to make sure that all audio plugins behave consistently when given invalid options.