Improve error handling and reporting

[acj]

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)
  • Improve error reporting when there's key info available (e.g. errno). Clean up TODO items along the way. I'm still getting my sea legs with Rust, so all feedback is welcome.
• what changes does this pull request make?
  • Bubble up errors related to invalid strings instead of unwrapping them
  • Include errno in error messages when possible
• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)
  • Returning a Result (e.g. from table and table_fd) will be a breaking change, as shown in the changes to the examples. Are we okay with that since this crate's status is experimental?

[brayniac]

Thanks for the PR. I'll have a chance to review it fully tomorrow. I think we'll be okay with breaking changes for this. I'll provide feedback in a review as soon as I have the time to look.

[brayniac]

Feedback in-line. Only concern is around the error types where noted. Otherwise, this looks good to me. Thanks for doing this!

[acj]

@brayniac Yes, good call on the error type. Fixed.

@javierhonduco Tests added. Good to see you over here :)

Thanks, both!