Errors with generators returning values should be TypeErrors.

js/rhino/src/org/mozilla/javascript/resources/Messages.properties

@@ -423,10 +423,10 @@ msg.return.inconsistent =\
   return statement is inconsistent with previous usage
 
 msg.generator.returns =\
-  generator function {0} returns a value
+  TypeError: generator function {0} returns a value
 
 msg.anon.generator.returns =\
-  anonymous generator function returns a value
+  TypeError: anonymous generator function returns a value
 
 msg.syntax =\
     syntax error