More error logging for tracking down mixer selem

Is your feature request related to a problem? Please describe. I've been attempting to setup spotifyd on a headless server connected to a bluetooth speaker; it's mostly working now, but I've been bashing my head against the wall trying to get volume controls working. Any mixer I select shows up in logs as unable to be found (Couldn't find selem with name ...).

Describe the solution you'd like Would it be possible to get more information in the log output as to which selems are available to spotifyd? eg. could it be possible to extend this log line to include something like:

format!(
    "Valid selems are: '{}'.",
    mixer.list_all_selms()
)

Describe alternatives you've considered Other alternatives might come down to documentation, eg. in the configuration docs for device you have:

# To get a list of valid devices, run `aplay -L`

Something similar for mixer could help!

That said, as far as I can tell, the mixers I am attempting to use do exist in alsa and just aren't able to get referenced properly by spotifyd(?), so this may not solve it.