Better handling if nix-command is not enabled

[ninjalf2]

After installing obelisk on a clean install of Ubuntu 24.10, I get this error when running ob run:

user@vm:~/emptydir$ ob run
./.obelisk/impl: command not cached, building ...
DONE Built on ./.obelisk/impl [command]
Process exited with code 1; /nix/store/7wfnj6hg24p9v212qfx81a16f6rnaqzy-nix-2.11.0/bin/nix eval --impure --expr $'(let a = import ./. {}; in toString (a.reflex.nixpkgs.lib.isDerivation a.passthru.staticFilesImpure))' --raw

Then, running the command myself I find out that it's because nix-command is not enabled:

user@vm:~/emptydir$ /nix/store/7wfnj6hg24p9v212qfx81a16f6rnaqzy-nix-2.11.0/bin/nix eval --impure --expr $'(let a = import ./. {}; in toString (a.reflex.nixpkgs.lib.isDerivation a.passthru.staticFilesImpure))' --raw
error: experimental Nix feature 'nix-command' is disabled; use '--extra-experimental-features

After setting experimental-features = nix-command in my /etc/nix/nix.conf and restarting the Nix daemon then it works and my local dev server is up and running. Great!

But the error message that I got initially was not very helpful and even though it did not take much work to find the cause, it should be detected somewhere automatically and the user should be handed an error message that actually explains what is wrong.

EDIT: Related to #1085, #1061, #1010, #1072, and possibly more. Sorry for not checking beforehand. Feel free to close it if necessary.