Warn or Error "guix time-machine" before July 2018

[zimoun]

Dear,

Trying old Python 3.6, I just checked the old commit with “git log” and
run the command:

--8<---------------cut here---------------start------------->8---
guix time-machine --commit=ce35dc84a1 \
    -- environment -C --ad-hoc python -- python --version
--8<---------------cut here---------------end--------------->8---

which after burning some CPU fails!

--8<---------------cut here---------------start------------->8---
[…]
  3507:20  4 (_)
   2806:4  3 (save-module-excursion #<procedure 7f935c0495d0 at ice-…>)
  3527:26  2 (_)
In unknown file:
           1 (primitive-load-path "gnu/packages/gnuzilla" #<procedur…>)
In gnu/packages/gnuzilla.scm:
   601:22  0 (_)

gnu/packages/gnuzilla.scm:601:22: ERROR:
  1. &message: "icecat-makeicecat.patch: patch not found"
--8<---------------cut here---------------end--------------->8---

Well, it is expected because the commit is from March 2018 so before the
big overhaul and all.  But it is annoying to hit it, especially if there
is some verification / authentication when “Updating channel”.

Does it make to add an error if the commit is not reachable for “guix
time-machine”?


All the best,
simon