When an include cannot be found, only print file path relative to source.

lib/jekyll/tags/include.rb

@@ -119,10 +119,14 @@ eos
         if safe && !realpath_prefixed_with?(path, dir)
           raise IOError.new "The included file '#{path}' should exist and should not be a symlink"
         elsif !File.exist?(path)
-          raise IOError.new "Included file '#{path}' not found"
+          raise IOError.new "Included file '#{path_relative_to_source(dir, path)}' not found"
         end
       end
 
+      def path_relative_to_source(dir, path)
+        File.join(INCLUDES_DIR, path.sub(Regexp.new("^#{dir}"), ""))
+      end
+
       def realpath_prefixed_with?(path, dir)
         File.exist?(path) && File.realpath(path).start_with?(dir)
       end