bug#38379: guix pull crashing: Throw to key `bad-response'

[Leo Famulari]

Today `guix pull` started crashing for me:

$ guix pull --verbosity=9 --substitute-urls=https://ci.guix.gnu.org
Updating channel 'guix' from Git repository at 
'https://git.savannah.gnu.org/git/guix.git'...
Building from this channel:
  guix      https://git.savannah.gnu.org/git/guix.git   5d297ca
Computing Guix derivation for 'x86_64-linux'... -
substitute: updating substitutes from 'https://ci.guix.gnu.org'...  
15.9%Backtrace:
substitute:            3 (apply-smob/1 #<catch-closure 7f82f190f1e0>)
substitute: In ice-9/boot-9.scm:
substitute:     705:2  2 (call-with-prompt _ _ #<procedure 
default-prompt-handler (k proc)>)
substitute: In ice-9/eval.scm:
substitute:     619:8  1 (_ #(#(#<directory (guile-user) 7f82f1585140>)))
substitute: In guix/ui.scm:
substitute:   1730:12  0 (run-guix-command _ . _)
substitute: 
substitute: guix/ui.scm:1730:12: In procedure run-guix-command:
substitute: Throw to key `bad-response' with args `("Bad Response-Line: ~s" 
(""))'.
guix pull: error: 
`/gnu/store/k76m7w6fmcy12iraz070sfw5ns410m42-guix-1.0.1-9.f63e493/bin/guix 
substitute' died unexpectedly

$ guix describe
Generation 102  Nov 04 2019 12:29:54    (current)
  guix bf7b08c
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: bf7b08c4fe7d14c25a83bde99f19eca1119d88ff