Handled unknown errors when preparing user message

- in the event we get an unknown error bubble up, we don't handle the
  templating on the error name
- `@tryghost/tpl` throws an error because we pass an undefined string:
  `Cannot read properties of undefined (reading 'replace')`
- this commit adds handling to fallback to a different user message in
  that event so we don't cause a 500 error

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

@@ -36,7 +36,8 @@ const messages = {
         HostLimitError: 'Host Limit error, cannot {action}.',
         DisabledFeatureError: 'Theme validation error, the {{{helperName}}} helper is not available. Cannot {action}.',
         UpdateCollisionError: 'Saving failed! Someone else is editing this post.'
-    }
+    },
+    UnknownError: 'Unknown error - {name}, cannot {action}.'
 };
 
 /**
@@ -161,7 +162,11 @@ const prepareUserMessage = (err, req) => {
                 userError.context = err.message;
             }
 
-            userError.message = tpl(messages.userMessages[err.name], {action: action});
+            if (_.get(messages.userMessages, err.name)) {
+                userError.message = tpl(messages.userMessages[err.name], {action: action});
+            } else {
+                userError.message = tpl(messages.UnknownError, {action, name: err.name});
+            }
         }
     }
 

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

@@ -2,7 +2,7 @@
 // const testUtils = require('./utils');
 require('./utils');
 const {InternalServerError} = require('@tryghost/errors');
-const {prepareError, handleJSONResponse, handleJSONResponseV2, handleHTMLResponse, prepareStack} = require('../lib/mw-error-handler');
+const {prepareError, handleJSONResponse, handleJSONResponseV2, handleHTMLResponse, prepareStack} = require('../');
 
 describe('Prepare Error', function () {
     it('Correctly prepares a normal error', function (done) {
@@ -56,6 +56,26 @@ describe('Error renderers', function () {
         }, () => {});
     });
 
+    it('Handles unknown errors when preparing user message', function (done) {
+        const errorRenderer = handleJSONResponseV2({
+            errorHandler: () => {}
+        })[3];
+
+        errorRenderer(new RangeError('test!'), {
+            frameOptions: {
+                docName: 'oembed',
+                method: 'read'
+            }
+        }, {
+            json: (data) => {
+                data.errors.length.should.eql(1);
+                data.errors[0].message.should.eql('Unknown error - RangeError, cannot read oembed.');
+                data.errors[0].context.should.eql('test!');
+                done();
+            }
+        }, () => {});
+    });
+
     it('Uses templates when required', function (done) {
         const errorRenderer = handleJSONResponseV2({
             errorHandler: () => {}