Moved `Error` in `repo.rs` out into a crate level `Error` in `error.rs`.
Repository specific errors are wrapped by the `crate::Error` as well.
Signed-off-by: Han Xu <keepsimple@gmail.com>
Initially, radicle-source was envisioned to be another layer on top
of radicle-surf, however, it ended up duplicating a lot of concepts.
We consolidate them by merging radicle-source into radicle-surf.
This meant removing modules branch.rs, tag.rs, oid.rs and error.rs.