(feat) Added Error variant and corresponding handlers.
(doc) Documented new Error variant.
(feat) Added chain_error to Chain and StackChain.

Fixes #106
(refactor) Refactored error-handling to work with automatic 404s and tests.
(doc) Small doc fixes.
Merge pull request #112 from reem/feat/error

(feat) Added real error handling through a new Error variant.