Removed calls to unwrap() and expect()

[Scipi]

For issue #33
This removes most calls to .unwrap() and .expect() except in example and test code as well as a handful of cases where we're guaranteed not to fail. Methods that were affected have had their signatures changed to return Result<T, Error>.

I implemented error types with thiserror so it's easy to wrangle together the different kinds of sub-errors that can occur under a common type. A downside here is it's more difficult to annotate certain common failures (like std::io::Error) with custom messages in certain places. It might be worth investigating using snafu in the future to assign contexts to errors, but for now there should be enough information in the errors themselves to describe the different points of failure.

[igiagkiozis]

Please don't send merge requests to master; use dev instead. Read https://github.com/igiagkiozis/plotly/blob/master/CONTRIBUTING.md.