fix: remote timeline client shutdown trips circuit breaker (#8495)

Before this PR

1.The circuit breaker would trip on CompactionError::Shutdown. That's
wrong, we want to ignore those cases.
2. remote timeline client shutdown would not be mapped to
CompactionError::Shutdown in all circumstances.

We observed this in staging, see
https://neondb.slack.com/archives/C033RQ5SPDH/p1721829745384449

This PR fixes (1) with a simple `match` statement, and (2) by switching
a bunch of `anyhow` usage over to distinguished errors that ultimately
get mapped to `CompactionError::Shutdown`.

I removed the implicit `#[from]` conversion from `anyhow::Error` to
`CompactionError::Other` to discover all the places that were mapping
remote timeline client shutdown to `anyhow::Error`.

In my opinion `#[from]` is an antipattern and we should avoid it,
especially for `anyhow::Error`. If some callee is going to return
anyhow, the very least the caller should to is to acknowledge, through a
`map_err(MyError::Other)` that they're conflating different failure
reasons.

pageserver/src/http/routes.rs

@@ -1650,7 +1650,9 @@ async fn timeline_compact_handler(
             .await
             .map_err(|e| ApiError::InternalServerError(e.into()))?;
         if wait_until_uploaded {
-            timeline.remote_client.wait_completion().await.map_err(ApiError::InternalServerError)?;
+            timeline.remote_client.wait_completion().await
+            // XXX map to correct ApiError for the cases where it's due to shutdown
+            .context("wait completion").map_err(ApiError::InternalServerError)?;
         }
         json_response(StatusCode::OK, ())
     }
@@ -1709,7 +1711,9 @@ async fn timeline_checkpoint_handler(
         }
 
         if wait_until_uploaded {
-            timeline.remote_client.wait_completion().await.map_err(ApiError::InternalServerError)?;
+            timeline.remote_client.wait_completion().await
+            // XXX map to correct ApiError for the cases where it's due to shutdown
+            .context("wait completion").map_err(ApiError::InternalServerError)?;
         }
 
         json_response(StatusCode::OK, ())