timeline_detach_ancestor: adjust error handling (#8528)

With additional phases from #8430 the `detach_ancestor::Error` became
untenable. Split it up into phases, and introduce laundering for
remaining `anyhow::Error` to propagate them as most often
`Error::ShuttingDown`.

Additionally, complete FIXMEs.

Cc: #6994

pageserver/src/http/routes.rs

@@ -1900,8 +1900,7 @@ async fn timeline_detach_ancestor_handler(
                         attempt,
                         ctx,
                     )
-                    .await
-                    .map_err(ApiError::InternalServerError)?;
+                    .await?;
 
                 AncestorDetached {
                     reparented_timelines,