Improve error handling (verbose mode?)

[bjgill]

See https://github.com/killercup/cargo-edit/pull/153#pullrequestreview-54449106

It would be nice to:

• [x] Switch to a more modern means of handling errors - error-chain instead of quick_error and Box<Error>
• [ ] Introduce some logging (toggled by a verbose mode, perhaps?)
• [ ] Have a good test suite for the CLI behaviour

waltz_cli seems like a good example of the quality bar we should be aiming for.

[bjgill]

As an added bonus, switching to error-chain will ensure that all our errors implement Send, which will make parallelisation (https://github.com/killercup/cargo-edit/issues/158) quite a bit easier.