FS-7019 detect debian explicitly, error on unknown distribution

debian/util.sh

@@ -390,9 +390,13 @@ build_all () {
       Ubuntu)
         distros="utopic trusty"
         ;;
-      *) # default to assuming Debian
+      Debian)
         distros="sid jessie wheezy"
         ;;
+      *)
+        echo "Unknown distribution"
+        exit -1
+        ;;
     esac
   fi
   ! $depinst || aptitude install -y \