Fixed error wording in fixture-utils

refs 404b2023e5
refs https://github.com/TryGhost/Toolbox/issues/129

- The previous error message wasn't helpful at all . Now we can tell "what?" fixture was looked-up and then go from there to find out why it is missing

test/utils/fixture-utils.js

@@ -711,7 +711,7 @@ const getFixtureOps = (toDos) => {
             });
         } else {
             if (!toDoList[toDo]) {
-                throw new Error('setup todo does not exist - spell mistake?');
+                throw new Error(`The fixture ${toDo} does not exist.`);
             }
 
             fixtureOps.push(toDoList[toDo]);