#595 - Fix issue with error icon not being shown in IE8.

Caused by an OpenLayers 2.10 bug where the loadend event isn't being generated when there are errors loading tiles in IE8.

Fixed by correcting the order of onImageLoadError and onerror event handler execution
Add some more comments explaining the why and how of the fix for #595
Incorporate review comments
Merge pull request #627 from aodn/issue-595

#595 - Fix issue with error icon not being shown in IE8.