- Fix ohm showing all fields as errored by default.
- Fix datamapper not showing errored fields.
Build common error specific pages (at least something for now):
403 - forbidden.
404 - not found.
500 - internal error.

@WayDotNet did the heavy lifting. I fixed:
- use a shared layout,
- remove javascript includes, there's no need for them,
- made CSS selector more specific (although perhaps we shouldn't even
  need to load an external stylesheet or maybe something other than
  admin!).
- remove "go back" text and leave the arrow only?