Hi guix,

This patch improves the error message generated by "guix time-machine"
and presumably "guix pull" as well when the guix derivation fails to
compute.

Before: something like <https://issues.guix.gnu.org/50232>.
After: something like

[...]
substitute: lijst van substitutes van ?[...]? aan het bijwerken... 100.0%
/gnu/store/4yx3vms0rfpzd1430za3g4pa6jvjsfw9-nowhere.tar.bz2.drv bouwen...
/error: build of `/gnu/store/6s0cmkfa4hq7d9w8g08mv2rhq0cr7wqc-subversion-1.14.1.drv' failed
guix time-machine: fout: You found a bug: the program '/gnu/store/j07lizpjf7v65shdgxyxddz171g6vjvx-compute-guix-derivation'
failed to compute the derivation for Guix (version: "9fc551e089c25f510f918a2c52917ba4e842f1c3"; system: "x86_64-linux";
host version: "75a3413b4e5c1f7443eb944a36ff364f4c4085f4"; pull-version: 1).
Please report it by email to <bug-guix@gnu.org

To test, apply the patch, and introduce an error in the hash of subversion:

;;; (gnu packages version-control)
(define-public subversion
  (package
    (name "subversion")
    (version "1.14.1")
    (source (origin
             (method url-fetch)
             ;; Oops!
             (uri "http://localhost/nowhere.tar.bz2")
             #;(string-append "mirror://apache/subversion/"
                                 "subversion-" version ".tar.bz2")
             (sha256
              (base32
               ;; Oops! 1->0
               #;"1ag1hvcm9q92kgalzbbgcsq9clxnzmbj9nciz9lmabjx4lyajp9c"
               "0ag1hvcm9q92kgalzbbgcsq9clxnzmbj9nciz9lmabjx4lyajp9c"))))
    (build-system gnu-build-system)
    [...]

and commit that mistake.  Then run
  guix time-machine --branch=master --url=$PWD --disable-authentication

Repeat without the error in subversion.

Greetings,
Maxime