add error-message if OS is unsuppored; fix indentation

cdist/conf/type/__package_update_index/explorer/currage

@@ -1,32 +1,17 @@
-#!/bin/sh
-#
-# 2018 Thomas Eckert (tom at it-eckert.de)
-#
-# This file is part of cdist.
-#
-# cdist is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-#
-# cdist is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with cdist. If not, see <http://www.gnu.org/licenses/>.
+#!/bin/sh -e
 
 os="$("$__explorer/os")"
 
 case "$os" in
-        debian|ubuntu|devuan)
-		if [ -f "/var/cache/apt/pkgcache.bin" ]; then
-			echo $(($(date +"%s")-$(stat --format '%Y' /var/cache/apt/pkgcache.bin)))
-		else
-			echo 0
-		fi
-                ;;
-        *)      ## not supported $os
-                ;;
+    debian|ubuntu|devuan)
+        if [ -f "/var/cache/apt/pkgcache.bin" ]; then
+            echo $(($(date +"%s")-$(stat --format '%Y' /var/cache/apt/pkgcache.bin)))
+        else
+            echo 0
+        fi
+        ;;
+    *)  echo "Your operating system ($os) is currently not supported by this type (${__type##*/})." >&2
+        echo "Please contribute an implementation for it if you can." >&2
+        exit 1
+        ;;
 esac