[Sass] Make sass --update use proper exit codes.

Closes gh-234

doc-src/SASS_CHANGELOG.md

@@ -13,6 +13,9 @@
 
 * Understand that mingw counts as Windows.
 
+* Make `sass --update` return a non-0 exit code if one or more files being updated
+  contained an error.
+
 ## 3.0.16
 
 [Tagged on GitHub](http://github.com/nex3/haml/commit/3.0.16).

lib/haml/exec.rb

@@ -422,15 +422,18 @@ MSG
           end
         end
 
+        had_error = false
         ::Sass::Plugin.on_creating_directory {|dirname| puts_action :directory, :green, dirname}
         ::Sass::Plugin.on_deleting_css {|filename| puts_action :delete, :yellow, filename}
         ::Sass::Plugin.on_compilation_error do |error, _, _|
           raise error unless error.is_a?(::Sass::SyntaxError)
+          had_error = true
           puts_action :error, :red, "#{error.sass_filename} (Line #{error.sass_line}: #{error.message})"
         end
 
         if @options[:update]
           ::Sass::Plugin.update_stylesheets(files)
+          exit 1 if had_error
           return
         end