tough: Reduce the Error Type's API Surface Area

[webern]

@iliana mentioned in #256

Opaque structs might solve the separate issue of our error variants being particularly purpose-specific, which makes stabilization more difficult.

Good point. The tough Error type is an enum with numerous variants. This presents a large API surface area for the error type which we should reduce significantly before v1.0.0.

Random thought, it seems weird that errors specific to RepositoryEditor would be in the same enum as errors unrelated to the RepositoryEditor, so there may be an opportunity there to use a different type for RepositoryEditor.

But, in general, we definitely need to hide a lot of the minutiae currently exposed by the tough error type in order to avoid frequent breaking changes.