Add a way to hook in to error handling when a page isn't found

[NullVoxPopuli]

A couple approaches:

• https://github.com/universal-ember/kolay/pull/82/files#diff-7d14e45cd0f6148d97f86a88a4f59c5055cf3b713c0b7213a0c8f596cdb3ca76R94
• or deriving data (preferred)

Regardless -- documentation should be added for various error-handling patterns