Switch from error-chain to failure for error handling

[sdleffler]

While working on Attaca I've been using the failure crate for error handling; for various reasons, it's vastly superior to the error-chain crate. Having to integrate error-chain with failure is proving to be problematic, because failure follows the philosophy that errors should be Send + Sync while error-chain maintains simply Send; this results in stuff like the failure::SyncFailure wrapper which puts a non-Sync error into a Mutex to make things safe. This is not cool and should be fixed. Migrating shouldn't be very difficult.