Improved routing on errors, proper error page.

This error page offers nav and suggestions focused on the main webapp.  If we want something special for curator pages, API errors, etc. we might need to update the routes_onerror (in SITE.routes.py) and possibly add error pages to other apps.
Merge pull request #100 from OpenTreeOfLife/simple-curation-app

Improved routing on errors, proper error page.