Audit errors/error handling

[kneitinger]

Currently each lib has decent error types, but their clarity/usage/etc. is inconsistent. All crate/mod error types should be reviewed and refined, and every occurrence of unwrap() should be checked and replaced unless completely certain a case is infallible.

Once the errors are refined, nightgraph-cli and nightgraph-ui should report errors cleanly to the user. ui can have an error console, or use the tab/panel/etc described in #6