Throw an Error rather than a string when createSpyObj is called incorectly

src/core/SpyFactory.js

@@ -54,7 +54,9 @@ getJasmineRequireObj().SpyFactory = function(j$) {
       }
 
       if (methods.length === 0 && properties.length === 0) {
-        throw 'createSpyObj requires a non-empty array or object of method names to create spies for';
+        throw new Error(
+          'createSpyObj requires a non-empty array or object of method names to create spies for'
+        );
       }
 
       return obj;