Make errors idiomatic

[MikkelPaulson]

Rust errors:

• Should be enums
• Should implement the Error trait
• Should implement Display

Most of the errors in the project, including the user-facing errors, satisfy none of these constraints. Refactor the various Result types, particularly the awkward and pervasive Result<T, Cow<str, 'static>>, to use proper enums that can bundle debugging information.