nixos/systemd: Custom error when mixing list/non-list defs

[roberth]

Motivation for this change

Provide context for an otherwise incomprehensible error and stack trace.

Before

error: value is a set while a list was expected

After

error: The definitions for systemd unit options should be either all lists, representing repeatable options, or all non-lists, but for the option systemd.services.postgresql.serviceConfig.ExecStartPre, the definitions are a mix of list and non-list
       - In `/nix/store/8nby8xsm4zf05nb1samj8gr8ya6xjq10-bi4hb1h745xlilain27syz9017a1a2l3-source/nixos/modules/system/boot/systemd.nix':
           [
             "/nix/store/74jkxldlh08njc5c8r24pw8rp5jmrvlw-unit-script-postgresql-pre-start/bin/postgresql-pre-start"
           ]
       - In `/home/user/proj/modules/postgresql.nix': <derivation /nix/store/bbd9zgis92iyaxn81m7l6zadb40rj0gd-postgres-standby-setup.drv>

This is a partial solution to #158606.
Ideally, I think we should have all systemd options modeled in NixOS, as this is not really a solution to the underlying problem, but it's not feasible until #158594 and perhaps even #158598 for performance (not sure).

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

cc @NixOS/systemd-committers

[roberth]

@NixOS/systemd please review

[Mic92]

Tested locally and works as intended.