Improve error handling (verbose mode?)

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`
- [ ] Introduce some logging (toggled by a verbose mode, perhaps?)
- [ ] Have a good test suite for the CLI behaviour

[waltz_cli](https://github.com/killercup/waltz/tree/ef79bf0fc3cfc8dd225cce864e21a91192121afe/waltz_cli) seems like a good example of the quality bar we should be aiming for.

Start with the linked pull-request review and inspect the CLI entry points responsible for error reporting. Compare the approach with the referenced waltz_cli example, then define the verbose-mode logging behavior and add coverage for the CLI behavior; the remaining checklist items should be complete when both are implemented and tested.

Written by the indexing model from the issue text.