bug#54259: pypi importer prints backtrace when no source package exists

[Ricardo Wurmus]

The only download for the kaleido package on Pypi is a binary.  There is
no source code on Pypi.  The importer recognizes this but then prints a
confusing backtrace when attempting to print a hint:

--8<---------------cut here---------------start------------->8---
./pre-inst-env guix import pypi kaleido
guix import: error: no source release for pypi package kaleido 0.2.1.post1

hint: Backtrace:
In ice-9/boot-9.scm:
  1752:10 18 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In unknown file:
          17 (apply-smob/0 #<thunk 7f6fab4442e0>)
In ice-9/boot-9.scm:
    724:2 16 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8 15 (_ #(#(#<directory (guile-user) 7f6fab449c80>)))
In guix/ui.scm:
   2229:7 14 (run-guix . _)
  2192:10 13 (run-guix-command _ . _)
In guix/scripts/import.scm:
   124:11 12 (guix-import . _)
In ice-9/boot-9.scm:
  1752:10 11 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In guix/scripts/import/pypi.scm:
    99:28 10 (_)
In guix/memoization.scm:
     98:0  9 (mproc "kaleido" #:version #f)
In unknown file:
           8 (_ #<procedure 7f6fa9f85380 at guix/memoization.scm:179:32 ()> 
#<procedure list _> (this is nothing))
In ice-9/boot-9.scm:
  1685:16  7 (raise-exception _ #:continuable? _)
In guix/ui.scm:
   806:16  6 (_ _)
    316:5  5 (display-hint _ _)
  1434:24  4 (texi->plain-text _)
In texinfo.scm:
  1132:22  3 (parse _)
   980:31  2 (loop #<input: string 7f6fa7652620> (*fragment*) _ _ _)
  1064:40  1 (_ _ _ (" " "source.  To build it from source, refer to the 
upstream repository at" " " "package is available on PyPI, but o…" …) …)
     92:2  0 (loop _ _ _ _)

texinfo.scm:92:2: In procedure loop:
Throw to key `parser-error' with args `(#f "@-command expected more arguments:" 
() (url #:opt title replacement) (url #:opt title replacement))'.
--8<---------------cut here---------------end--------------->8---

-- 
Ricardo