Bug#661501: debootstrap: Missing keyring file should abort with error

[jnqnfe <jnqnfe@gmail.com>]

Updated 1.0.67 based patch attached.

I am not currently particularly familiar with the debootstrap codebase,
however this certainly seems like a condition that should result in an
error to me.

commit 8e084dc9b0c6d6ee02668e3581e55bbe3047a4d1
Author: jnqnfe <jnqnfe@gmail.com>
Date:   Thu Jan 15 18:05:20 2015 +0000

    Issue error instead of warning if keyring file missing

diff --git a/functions b/functions
index bf999f1..c94faf8 100644
--- a/functions
+++ b/functions
@@ -204,7 +204,7 @@ keyring () {
 				info KEYRING "Keyring file not available at %s; switching to https mirror %s" "$1" "$DEF_HTTPS_MIRROR"
 				USER_MIRROR="$DEF_HTTPS_MIRROR"
 			else
-				warning KEYRING "Cannot check Release signature; keyring file not available %s" "$1"
+				error 1 KEYRING "Cannot check Release signature; keyring file not available %s" "$1"
 			fi
 		fi
 	fi