Move to a more scalable errors implementation

[mp4096]

The current errors implementation is hand-written, with half-hearted conversions and formatting implementations. If we want to add more features to indentex, we should find a scalable, boilerplate-free method of error handling.

A quick search on crates.io reveals two primary candidates:

• quick-error
• error-chain

Both contestants have a lot of dependent crates, e.g. handlebars uses quick-chain, tera uses error-chain.

I'm not sure which one to use, however I intuitively tend to error-chain.

[mp4096]

error-chain has been moved to rust-lang-nursery, so this is kind of a serious 👍 for it.