pageserver: don't squash all image layer generation errors into anyhow::Error (#7943)

## Problem

CreateImageLayersError and CompactionError had proper From
implementations, but compact_legacy was explicitly squashing all image
layer errors into an anyhow::Error anyway.

This led to errors like:
```
 Error processing HTTP request: InternalServerError(timeline shutting down

Stack backtrace:
   0: <<anyhow::Error as core::convert::From<pageserver::tenant::timeline::CreateImageLayersError>>::from as core::ops::function::FnOnce<(pageserver::tenant::timeline::CreateImageLayersError,)>>::call_once
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/ops/function.rs:250:5
   1: <core::result::Result<alloc::vec::Vec<pageserver::tenant::storage_layer::layer::ResidentLayer>, pageserver::tenant::timeline::CreateImageLayersError>>::map_err::<anyhow::Error, <anyhow::Error as core::convert::From<pageserver::tenant::timeline::CreateImageLayersError>>::from>
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/result.rs:829:27
   2: <pageserver::tenant::timeline::Timeline>::compact_legacy::{closure#0}
             at pageserver/src/tenant/timeline/compaction.rs:125:36
   3: <pageserver::tenant::timeline::Timeline>::compact::{closure#0}
             at pageserver/src/tenant/timeline.rs:1719:84
   4: pageserver::http::routes::timeline_checkpoint_handler::{closure#0}::{closure#0}
```

Closes: https://github.com/neondatabase/neon/issues/7861

pageserver/src/tenant/timeline/compaction.rs

@@ -133,8 +133,7 @@ impl Timeline {
                         },
                         &image_ctx,
                     )
-                    .await
-                    .map_err(anyhow::Error::from)?;
+                    .await?;
 
                 self.upload_new_image_layers(image_layers)?;
                 partitioning.parts.len()