update error handling to allow path to creating a tiddler
update error handling to allow path to creating a tiddler

this makes the 404 page more useful by allowing a user to create
a tiddler from a 404 page.

e.g. navigate to http://jon.tiddlyspace.com/Jon Tiddler
create a link to create "Jon Tiddler" in takenote

TiddlySpace/tiddlyspace,FND/tiddlyspace,TiddlySpace/tiddlyspace,FND/tiddlyspace,FND/tiddlyspace,TiddlySpace/tiddlyspace