Return 404 instead of 500 from the gem proxy when asked for files it doesn't have

This fixes URLs like http://rubygems-proxy.torquebox.org/releases/rubygems/warbler/1.4.1.dev/warbler-1.4.1.dev.pom to return a 404 instead of a 500. This change is already deployed to rubygems-proxy.torquebox.org.