BREAKING: Tree interface functions now return errors

Have the calls that can fail return the error interface, instead just a bool indicating
success.

This removes any previously deprecated functions, such as `(*Tree).Set` (going forward,
use `(*Tree).SetType`), or `IsConfigAlreadyLoaded` (use `errors.Is`/`As` from the Go
standard library instead).

Also some minor tweaks for readability and adhering to conventions.