bug#52350: pypi import: ungraceful exit when recursively importing unknown package

[Ricardo Wurmus <rekado@elephly.net>]

When using the pypi importer, a typo in the package name leads to an
ugly backtrace:

--8<---------------cut here---------------start------------->8---
./pre-inst-env guix import pypi -r pytestrerunfailures
following redirection to `https://pypi.org/pypi/pytestrerunfailures/json/'...
Backtrace:
In ice-9/boot-9.scm:
  1752:10  9 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In unknown file:
           8 (apply-smob/0 #<thunk 7f3f0d996f60>)
In ice-9/boot-9.scm:
    724:2  7 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8  6 (_ #(#(#<directory (guile-user) 7f3f0d990c80>)))
In guix/ui.scm:
   2206:7  5 (run-guix . _)
  2169:10  4 (run-guix-command _ . _)
In guix/scripts/import.scm:
   124:11  3 (guix-import . _)
In guix/scripts/import/pypi.scm:
    96:18  2 (guix-import-pypi . _)
In guix/import/utils.scm:
   498:27  1 (recursive-import "pytestrerunfailures" #:repo->guix-package _ #:guix-name _ …)
   488:33  0 (lookup-node "pytestrerunfailures" #f)

guix/import/utils.scm:488:33: In procedure lookup-node:
Wrong number of values returned to continuation (expected 2)
--8<---------------cut here---------------end--------------->8---

-- 
Ricardo