Refactored error cache control logic to middleware

refs https://github.com/TryGhost/Toolbox/issues/410

- This is groundwork for split cache-control rules for Admin app endpoints and the rest of Ghost apps.

ghost/core/core/frontend/web/middleware/error-handler.js

@@ -7,7 +7,7 @@ const config = require('../../../shared/config');
 const renderer = require('../../services/rendering');
 
 // @TODO: make this properly shared code
-const {prepareError, prepareStack} = require('@tryghost/mw-error-handler');
+const {prepareError, prepareErrorCacheControl, prepareStack} = require('@tryghost/mw-error-handler');
 
 const messages = {
     oopsErrorTemplateHasError: 'Oops, seems there is an error in the error template.',
@@ -86,6 +86,8 @@ const themeErrorRenderer = (err, req, res, next) => {
 module.exports.handleThemeResponse = [
     // Make sure the error can be served
     prepareError,
+    // Add cache-control header
+    prepareErrorCacheControl,
     // Handle the error in Sentry
     sentry.errorHandler,
     // Format the stack for the user

ghost/mw-error-handler/lib/mw-error-handler.js

@@ -90,11 +90,6 @@ module.exports.prepareError = (err, req, res, next) => {
     // alternative for res.status();
     res.statusCode = err.statusCode;
 
-    // never cache errors
-    res.set({
-        'Cache-Control': 'no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0'
-    });
-
     next(err);
 };
 
@@ -122,6 +117,17 @@ const jsonErrorRenderer = (err, req, res, next) => { // eslint-disable-line no-u
     });
 };
 
+module.exports.prepareErrorCacheControl = (err, req, res, next) => {
+    // never cache errors
+    let cacheControl = 'no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0';
+
+    res.set({
+        'Cache-Control': cacheControl
+    });
+
+    next(err);
+};
+
 const prepareUserMessage = (err, req) => {
     const userError = {
         message: err.message,
@@ -221,6 +227,8 @@ module.exports.pageNotFound = (req, res, next) => {
 module.exports.handleJSONResponse = sentry => [
     // Make sure the error can be served
     module.exports.prepareError,
+    // Add cache-control header
+    module.exports.prepareErrorCacheControl,
     // Handle the error in Sentry
     sentry.errorHandler,
     // Format the stack for the user