Add new error if user likes their own post

lib/errors.js

@@ -9,6 +9,7 @@ let Errors = {
 	invalidToken: 'The token provided was not valid',
 	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',
 	invalidParameter (param, message) {
 		if(message) {
 			var punctuatedMessage = ': ' + message