stage-1-init: better warnings
stage-1-init: better warnings

- Suppress spurious LVM FD3 warning:
  See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=432986
- Warn when device doesn't come online

NixOS/nixpkgs,SymbiFlow/nixpkgs,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,triton/triton,triton/triton,triton/triton,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,triton/triton,NixOS/nixpkgs,triton/triton,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,triton/triton,NixOS/nixpkgs,SymbiFlow/nixpkgs