More error logging for tracking down mixer selem

[TheKevJames]

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](https://docs.spotifyd.rs/config/File.html) 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.