Use a boolean 'special' to identify special pages

Use a boolean 'special' to identify special pages

We used to load special pages, such as error pages as 'blank'
pages internally and try to identify them indirectly. This
breaks navigating back to and forward to such pages and makes
it difficult to allow stock icons for them.

Now a boolean 'special' is used which is reset on any
navigation attempt and any finished load. It is set only
if a special page is loaded.

This should make icons in error pages work reliably.