Add check for missing onclick function, document, fix exception

* Throws an exception if you use action: okay without an onclick
* Document some action-related things more clearly
* Have exception class work around https://code.google.com/p/chromium/issues/detail?id=228909

Change-Id: I1003825a40498c1f71c7ece6bf656c6bf8f60728
Merge "Add check for missing onclick function, document, fix exception"