nixos/filesystems: guard assertion messages against eval failures

[philiptaron]

Description of changes

The messages for these filesystem assertions ought to be able to be printed even if the assertion condition is not encountered.

I encountered this while testing #313497.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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
• Fits CONTRIBUTING.md.

[ElvishJerricco]

@philiptaron I really dislike this method of repeating the condition in the message to replace the message with a more nonsense one. I think it would have been better for this to have simply altered the behavior of the assertions option, maybe with an apply setting for the type of message, so that it replaces the message with "assertion not triggered" or something.