add 404-specific error message

we trap 404s already because we want to bail out early on 404s, so
instead of displaying the typical bug report, display a more useful
banner to herd users in the right direction.
text fixes to 404 error
drop mid-Dec and wordsmith a bit.

make it a bit more obvious that the scary paragraph in the middle is
for hackers.
Merge pull request #68 from opscode/lcg/better-404-error

add 404-specific error message