Add threadLocked error

lib/errors.js

@@ -10,6 +10,7 @@ let Errors = {
 	noSettings: 'You haven\'t added any settings yet',
 	passwordSame: 'You can\'t set it to the same password',
 	cannotLikeOwnPost: 'You can\'t like your own post',
+	threadLocked: 'You can\'t post to a locked thread',
 	invalidParameter (param, message) {
 		if(message) {
 			var punctuatedMessage = ': ' + message