pkgs/top-level/aliases.nix: improve error messages
pkgs/top-level/aliases.nix: improve error messages

Change error messages to start with or at least mention the name of the
package being referenced. This avoids obscure error messages like
"error: Abandoned by upstream." when rebuilding your system.
Also do some trivial cosmetic changes while touching things.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs