Correct error handling.

If something goes wrong running the requirejs-config file, we would
previously just crash with an unhandled promise rejection.

Now we properly surface the error.

Author: toddbc

index.js

@@ -65,7 +65,9 @@ RequireJsResolverPlugin.prototype.apply = function(resolver) {
             } else {
                 callback();
             }
-        })
+        }, err => {
+            callback(err);
+        });
     });
 };