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

libs/remote_storage/src/error.rs

@@ -42,6 +42,10 @@ impl DownloadError {
             Timeout | Other(_) => false,
         }
     }
+
+    pub fn is_cancelled(&self) -> bool {
+        matches!(self, DownloadError::Cancelled)
+    }
 }
 
 impl From<std::io::Error> for DownloadError {