Improve prettiness of 4xx error handling.

This intercepts the exceptions with some template handling
and some (silly) CSS.

This works on #69 but does not complete it. There needs to be
additional text.

@FND might like to see this because it a) steals from his stuff
https://github.com/FND/tiddlywebplugins.bfw/pull/6/files
b) has a way to handle 405s.
Merge branch 'master' of github.com:cdent/tank