Rework remaining errors

[charlespierce]

Closes #296
Closes #298
Closes #299
Closes #300
Closes #305
Closes #343

Info

• Several of the modules are interconnected and updating the errors in the fs module led to updating many of the other modules.

Changes

• Refactored the fs module to not use .unknown() but instead return an io::Error, as they are still IO utilities.
• Updated distro module to convert .unknown() into .with_context()
• Updated hook module to convert .unknown() into .with_context()
• Updated inventory module to convert .unknown() into .with_context()
• Updated toolchain module to convert .unknown() into .with_context()

Notes

• There is a lot of duplication in Read / Write errors. It is a common pattern to call path.to_string_lossy().to_string() to convert a Path / PathBuf to a string. I'm not sure the best approach to refactor that to prevent having that same block appear in multiple places.

[chriskrycho]

A couple small changes – most of the comments here are things we should defer to a cleanup PR so we can land this one and not block other things any longer.

[charlespierce]

Updated to address most of the comments, still investigating passing an owned PathBuf to the error object, instead of a String.