#57 Handle case where outline ID is not found in reference file

[Swami Kevala]

If a document contains any markup elements which have IDs which are not in the reference file (either because the markupType was removed - or modified by hacking the reference file), a message should be displayed informing the user what has happened. (Could give the missing ID code as well as the associated block IDs).

The file should then open properly - leaving out the affected markup element.

Currently no message is displayed, and weird things happen to the tree display, and the text is not displayed at all.