Schedule the pageLoadNotTriggered timer *after* script execution.

If the target function loads the page, then this will be completed by the
time pageLoadNotTriggered is scheduled. So the effect is to make
_execFuncAndWaitForLoadDecorator more deterministic (since any synchronous
loading will definitely be detected and handled correctly).

Tidy up comments:
* Clarify some of the circumstances in which onErrorFunc might be called.
* Fix mistakes in comment.