working_copy: drop "Internal backend error" message from error

I don't think the message adds anything over what the `BackendError`
itself provides, so let's use `transparent` instead.

I also dropped the `Internal` prefix from the variant because that
also didn't seem to add anything.

lib/src/working_copy.rs

···
         path: PathBuf,
     },
     /// Reading or writing from the commit backend failed.
-    #[error("Internal backend error")]
-    InternalBackendError(#[from] BackendError),
+    #[error(transparent)]
+    BackendError(#[from] BackendError),
     /// A file was larger than the specified maximum file size for new
     /// (previously untracked) files.
     #[error("New file {path} of size ~{size} exceeds snapshot.max-new-file-size ({max_size})")]