Changed app/loader to use @tryghost/errors

- getting rid of instances of new Error as we should always use @tryghost/errors
- Whilst here, got rid of i18n but discovered the messages were missing!
- This is my fault, they disappeared when I removed external apps and clearly removed too much: 8c1a0b8d0c (diff-0f5cc40aa8906a1be1bad2002a35361bbf9e766e46b3b29be10f4f479265426a)
- Therefore, I have restored these messages in the places where they were used, except amp_content, where I have written a new message, as the message that was there was not relevant

core/frontend/apps/amp/lib/helpers/amp_content.js

@@ -9,7 +9,7 @@
 const Promise = require('bluebird');
 
 const moment = require('moment');
-const {SafeString, logging, i18n, errors} = require('../../../../services/proxy');
+const {SafeString, logging, errors} = require('../../../../services/proxy');
 const amperizeCache = {};
 let allowedAMPTags = [];
 let allowedAMPAttributes = {};
@@ -128,11 +128,11 @@ function getAmperizeHTML(html, post) {
                     if (err.src) {
                         // This is a valid 500 GhostError because it means the amperize parser is unable to handle some Ghost HTML.
                         logging.error(new errors.GhostError({
-                            message: `AMP HTML couldn't get parsed: ${err.src}`,
+                            message: `AMP HTML couldn't be parsed: ${err.src}`,
                             code: 'AMP_PARSER_ERROR',
                             err: err,
                             context: post.url,
-                            help: i18n.t('errors.apps.appWillNotBeLoaded.help')
+                            help: 'Please share this error on GitHub or https://forum.ghost.org'
                         }));
                     } else {
                         logging.error(new errors.GhostError({err, code: 'AMP_PARSER_ERROR'}));

core/frontend/services/apps/index.js

@@ -1,11 +1,16 @@
 const debug = require('@tryghost/debug')('services:apps');
 const Promise = require('bluebird');
-const i18n = require('../../../shared/i18n');
+const tpl = require('@tryghost/tpl');
 const logging = require('@tryghost/logging');
 const errors = require('@tryghost/errors');
 const config = require('../../../shared/config');
 const loader = require('./loader');
 
+const messages = {
+    appWillNotBeLoadedError: 'The app will not be loaded',
+    appWillNotBeLoadedHelp: 'Check with the app creator, or read the app documentation for more details on app requirements'
+};
+
 module.exports = {
     init: function () {
         debug('init begin');
@@ -15,8 +20,8 @@ module.exports = {
             .catch(function (err) {
                 logging.error(new errors.GhostError({
                     err: err,
-                    context: i18n.t('errors.apps.appWillNotBeLoaded.error'),
+                    context: tpl(messages.appWillNotBeLoadedError),
-                    help: i18n.t('errors.apps.appWillNotBeLoaded.help')
+                    help: tpl(messages.appWillNotBeLoadedHelp)
                 }));
             });
     }

core/frontend/services/apps/loader.js

@@ -1,10 +1,15 @@
 const path = require('path');
 const _ = require('lodash');
 const Promise = require('bluebird');
-const i18n = require('../../../shared/i18n');
+const errors = require('@tryghost/errors');
+const tpl = require('@tryghost/tpl');
 const config = require('../../../shared/config');
 const Proxy = require('./proxy');
 
+const messages = {
+    noActivateMethodLoadingAppError: 'Error loading app named {name}; no activate() method defined.'
+};
+
 // Get the full path to an app by name
 function getAppAbsolutePath(name) {
     return path.join(config.get('paths').internalAppPath, name);
@@ -35,7 +40,9 @@ module.exports = {
 
         // Check for an activate() method on the app.
         if (!_.isFunction(app.activate)) {
-            return Promise.reject(new Error(i18n.t('errors.apps.noActivateMethodLoadingApp.error', {name: name})));
+            return Promise.reject(new errors.IncorrectUsageError(
+                tpl(messages.noActivateMethodLoadingAppError, {name: name})
+            ));
         }
 
         // Wrapping the activate() with a when because it's possible