magic-proxy: dump proxy log to stdout on failure (LP: #1847300)

When we encounter a failure in 'lb binary' the launchpad builders can
only surface the build output from stdout.  If the binary hook failure
implicates the archive we can not determine fault without the apt
proxy log.  This patch will dump the proxy log to stdout to aid in
debugging these failures.

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.408.54ubuntu1) UNRELEASED; urgency=medium
+
+  * magic-proxy: dump proxy log to stdout on failure (LP: #1847300)
+
+ -- Robert C Jennings <robert.jennings@canonical.com>  Tue, 08 Oct 2019 11:02:08 -0500
+
 livecd-rootfs (2.408.54) xenial; urgency=medium
 
   * Revert exclusion of makedev from buildd chroots, as it turned out not to

live-build/auto/build

@@ -393,6 +393,12 @@ EOF
 if [ -e binary.success ]; then
 	rm -f binary.success
 else
+	# Dump the magic-proxy log to stdout on failure to aid debugging
+	if [ -f /build/livecd.magic-proxy.log ] ; then
+		echo "================= Magic proxy log (start) ================="
+		cat /build/livecd.magic-proxy.log
+		echo "================== Magic proxy log (end) =================="
+	fi
 	exit 1
 fi