Description
currently when no x-server is available (or other dbus errors) we just panic, but this could just be a warning and then continue without dbus.

To Reproduce

1. compile with dbus enables d
2. run without an x-server (i thing unsetting DISPLAY should also work).
3. run spotifyd normally
4. crashes with a panic

Expected behavior
a warning, but not a panic

Logs
see #729 for a relevant issue.

Compilation flags

• dbus_mpris
• dbus_keyring
• alsa_backend
• portaudio_backend
• pulseaudio_backend
• rodio_backend

a quicker solution might be to provide a command line option to disable dbus.

relevant code locations:

• creating a dbus instance

  spotifyd/src/main_loop.rs

  Line 182 in 8cd1240

  self.spotifyd_state.dbus_mpris_server = new_dbus_server(

  
  this could be put behind an if triggered by a command line option, thus avoiding the panic.

• spotifyd/src/dbus_mpris.rs

  Line 175 in a863582

  Connection::get_private(BusType::Session).expect("Failed to initialize DBus connection"),

  
  this is where the panic happens, instead there could be a logger warning, and return something else