Refactor error handling

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Been bugging me for a while now, and now seemed like a good time to change because less conflicts
  • Switches all errors to derive std::error::Error through the thiserror crate (https://github.com/dtolnay/thiserror) which makes it more extensible and clean/quick to use

There are still things I think could be improved, but I tried to keep these changes unopinionated without making any functional changes.

Only functional changes are two things I suspect to be bugs here if that doesn't resolve, the change(s) in chain_store.rs @dutterbutter can you confirm there isn't a specific reason it should not change

Reference issue to close (if applicable)

Closes
#216

Other information and links

[dutterbutter]

@austinabell yes that should return Ok(()) rather than an error. Nice catch!