Friendlier error message when a non-default layout is not in the layouts directory.

middleman-core/lib/middleman-core/core_extensions/rendering.rb

@@ -115,9 +115,13 @@ module Middleman
           # handles cases like `style.css.sass.erb`
           content = nil
           while ::Tilt[path]
+            begin
               opts[:template_body] = content if content
               content = render_individual_file(path, locs, opts, context)
               path = File.basename(path, File.extname(path))
+            rescue LocalJumpError => e
+              raise "Tried to render a layout (calls yield) at #{path} like it was a template. Non-default layouts need to be in #{source}/layouts."
+            end
           end
       
           # Certain output file types don't use layouts