Hello there!

Org provides no good way to ignore broken links when exporting subtrees
with lots of links.  With `org-export-with-broken-links' set to `nil',
the exporter aborts.  When set to `t', the exporter removes the linked
words altogether, breaking the sentences.  Lastly, when set to `mark',
the sentences become unreadable due to the million [BROKEN LINK: ...]
fragments everywhere.

What do you folks think about the WIP patch below?  It allows the user
to customize the broken link marker the Org exporter will use.  A user
like me, one who works in a large Org file and often exports only its
parts, can use a more gentle marker or even no marker at all.