Fix needless-return clippy lints and switch to thiserror

[bbhtt]

No description provided.

[bbhtt]

failure is archived since 2021.

error: non-local `impl` definition, `impl` blocks should be written at the same level as their item
  --> src/errors.rs:10:10
   |
10 | #[derive(Fail, Debug, Clone)]
   |          ^---
   |          |
   |          `Fail` is not local
   |          move the `impl` block outside of this constant `_DERIVE_failure_Fail_FOR_DeltaGenerationError`