Patch ActionDispatch::DebugExceptions to capture NotFound exceptions

[wjordan]

This PR patches ActionDispatch::DebugExceptions to capture those pesky hard-coded ActionController::RoutingError exceptions generated by non-route-matching X-Cascade == 'pass' responses, re-publishing them to the ActiveSupport::Notifications bus where they are cleanly logged by Lograge, instead of automatically writing a full exception backtrace directly to the Rails log.

Fixes #27, #142.

~No tests yet other than running manually in my project- could use guidance on the best way to cover this.~

[iloveitaly]

@wjordan this looks like a great change! Can you rebase on master so the latest CI will run?