Fix typo in variable: execexecTypeOpt -> execTypeOpt.
Stop the page before trying to load a new URL, otherwise our callbacks get confused.
Set pageLoadNotTriggered in onLoadFinished as well.
This is a defensive step, in case there is a catastrophic mixup with the event handlers.
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.
Merge pull request #147 from garyd203/page-load-cleanup

_execFuncAndWaitForLoadDecorator cleanup