Add postRemoved error

lib/errors.js

@@ -11,6 +11,7 @@ let Errors = {
 	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',
+	postRemoved: 'You can\'t reply to a removed post',
 	invalidParameter (param, message) {
 		if(message) {
 			var punctuatedMessage = ': ' + message