New `devmode` hides build errors about nix.dev

[asymmetric]

This was fixed in #811

[asymmetric]

cc @alejandrosame @fricklerhandwerk since you worked on this in 42add65.

[asymmetric]

I also tried seeing if the call to shell could be made to distinguish between errors and success, but I couldn't find a way to distinguish between normal output/warnings and errors (this might be because the nix command prints stuff to stderr when it shouldn't, not sure).

[asymmetric]

Also, IME it's quite a bit slower than the previous version, probably because it goes through a whole nix build rather than just calling sphinx-bulid.

[alejandrosame]

@asymmetric Please try PR #788. The error now gets rendered and displayed in the browser.

At the moment we introduced this devmode I wanted to reuse somehow the manuals utility, since there was a considerable effort to make it work resiliently. This problem probably justifies the copy-pasting in the PR until the utility is accessible from outside of nixpkgs.