git-download: 'git-fetch' really returns #f upon error.

This allows the fallback code in (guix git-download) to actually run.
Regression introduced in commit 329dabe13b.

Fixes <https://bugs.gnu.org/33911>.
Reported by Björn Höfling <bjoern.hoefling@bjoernhoefling.de>.

* guix/build/git.scm (git-fetch): Guard against 'invoke-error?' and
really return #f upon failure.

guix/build/git.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2016, 2019 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -18,6 +18,8 @@
 
 (define-module (guix build git)
   #:use-module (guix build utils)
+  #:use-module (srfi srfi-34)
+  #:use-module (ice-9 format)
   #:export (git-fetch))
 
 ;;; Commentary:
@@ -39,6 +41,16 @@ recursively.  Return #t on success, #f otherwise."
 
   (mkdir-p directory)
 
+  (guard (c ((invoke-error? c)
+             (format (current-error-port)
+                     "git-fetch: '~a~{ ~a~}' failed with exit code ~a~%"
+                     (invoke-error-program c)
+                     (invoke-error-arguments c)
+                     (or (invoke-error-exit-status c) ;XXX: not quite accurate
+                         (invoke-error-stop-signal c)
+                         (invoke-error-term-signal c)))
+             (delete-file-recursively directory)
+             #f))
     (with-directory-excursion directory
       (invoke git-command "init")
       (invoke git-command "remote" "add" "origin" url)
@@ -64,6 +76,6 @@ recursively.  Return #t on success, #f otherwise."
       ;; status of the Git repo.  Since we want a fixed output, this
       ;; directory needs to be taken out.
       (delete-file-recursively ".git")
-      #t))
+      #t)))
 
 ;;; git.scm ends here