Add Layout#relative_path so the layout can tell us what its path is for error messages

lib/jekyll/convertible.rb

@@ -217,7 +217,7 @@ module Jekyll
         self.output = render_liquid(layout.content,
                                     payload,
                                     info,
-                                         File.join(site.config['layouts_dir'], layout.name))
+                                    layout.relative_path)
 
         # Add layout to dependency tree
         site.regenerator.add_dependency(

lib/jekyll/layout.rb

@@ -45,5 +45,13 @@ module Jekyll
     def process(name)
       self.ext = File.extname(name)
     end
+
+    # The path to the layout, relative to the site source.
+    #
+    # Returns a String path which represents the relative path
+    #   from the site source to this layout
+    def relative_path
+      @relative_path ||= Pathname.new(path).relative_path_from(Pathname.new(site.source)).to_s
+    end
   end
 end

lib/jekyll/renderer.rb

@@ -146,7 +146,7 @@ module Jekyll
           layout.content,
           payload,
           info,
-          File.join(site.config['layouts_dir'], layout.name)
+          layout.relative_path
         )
 
         # Add layout to dependency tree