Have the build fail when spelling mistakes are found.

Rakefile

@@ -29,6 +29,8 @@ task :spellcheck do
     io.puts(File.read("wordlist"))
   end
 
+  errors = false
+
   Dir["**/*.md"].each do |file|
     command = %q{
       ruby -pe 'gsub /^    .*$/, ""' |
@@ -41,8 +43,13 @@ task :spellcheck do
       line[/^& ([^ ]+)/, 1]
     end.compact
 
-    puts "#{file}: #{words.uniq.sort.join(" ")}" if words.any?
+    if words.size > 0
+      errors = true
+      puts("#{file}: #{words.uniq.sort.join(" ")}")
+    end
   end
+
+  abort("Spelling errors found.") if errors
 end
 
 namespace :format do