Added error handling for weird handlebars syntax

refs #10496

- handlebars if and unless helpers throw weird, unhelpful syntax errors
- for now, catch these errors and do something helpful with them

core/server/web/middleware/error-handler.js

@@ -38,6 +38,14 @@ _private.prepareError = function prepareError(err, req, res, next) {
             err = new common.errors.NotFoundError({
                 err: err
             });
+        } else if (err instanceof TypeError && err.stack.match(/node_modules\/handlebars\//)) {
+            // Temporary handling of theme errors from handlebars
+            // @TODO remove this when #10496 is solved properly
+            err = new common.errors.IncorrectUsageError({
+                err: err,
+                message: '{{#if}} or {{#unless}} helper is malformed',
+                statusCode: err.statusCode
+            });
         } else {
             err = new common.errors.GhostError({
                 err: err,