Bug 24427 - make all errors asynchronous

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24427

This bug asked the question of whether all errors should be made
asynchronous, and reported through the Promise rather than as an exception
to the method call.

There is so far one comment supporting this change and no other comments.

If there are no further comments I will implement this change shortly.

...Mark