Fix an undefined reference error and make log message clearer

I'm not picky about the actual message, but I encoutered two problems that lead to this change:
 1.  First I was hitting a ReferenceError on msg that was swallowing the message from being returned to user code.
 2.  When I fixed that I realized the old log message wouldn't tell you which product was attempting to be purchased.  e.g.

```
   storekit.load(['com.foo.product1'])
   ...
   storekit.purchase('bogus', 1)
```

Should now report an error like "Purchasing bogus failed.  Ensure the product was loaded first with storekit.load(...)!"