Moved error messages to "messages" hash

refs https://linear.app/tryghost/issue/CORE-49/fix-errors-in-utils-repo-limit-service

- As I've touched these files did a little refactor and changed where the error messages are stored to keep it up with our lates coding standard - having "messages" hash defined in the module storing all messages that have pottential for i18y in the future.

ghost/limit-service/lib/date-utils.js

@@ -1,6 +1,10 @@
 const {DateTime} = require('luxon');
 const {IncorrectUsageError} = require('@tryghost/errors');
 
+const messages = {
+    invalidInterval: 'Invalid interval specified. Only "month" value is accepted.'
+};
+
 const SUPPORTED_INTERVALS = ['month'];
 /**
  * Calculates the start of the last period (billing, cycle, etc.) based on the start date
@@ -23,7 +27,7 @@ const lastPeriodStart = (startDate, interval) => {
     }
 
     throw new IncorrectUsageError({
-        message: 'Invalid interval specified. Only "month" value is accepted.'
+        message: messages.invalidInterval
     });
 };
 

ghost/limit-service/lib/limit-service.js

@@ -3,6 +3,11 @@ const config = require('./config');
 const {IncorrectUsageError} = require('@tryghost/errors');
 const _ = require('lodash');
 
+const messages = {
+    missingErrorsConfig: `Config Missing: 'errors' is required.`,
+    noSubscriptionParameter: 'Attempted to setup a periodic max limit without a subscription'
+};
+
 class LimitService {
     constructor() {
         this.limits = {};
@@ -21,7 +26,7 @@ class LimitService {
     loadLimits({limits = {}, subscription, helpLink, db, errors}) {
         if (!errors) {
             throw new IncorrectUsageError({
-                message: `Config Missing: 'errors' is required.`
+                message: messages.missingErrorsConfig
             });
         }
 
@@ -44,8 +49,8 @@ class LimitService {
                     this.limits[name] = new MaxLimit({name: name, config: limitConfig, helpLink, db, errors});
                 } else if (_.has(limitConfig, 'maxPeriodic')) {
                     if (subscription === undefined) {
-                        throw new errors.IncorrectUsageError({
-                            message: 'Attempted to setup a periodic max limit without a subscription'
+                        throw new IncorrectUsageError({
+                            message: messages.noSubscriptionParameter
                         });
                     }