review: proper error handling for OwnedAsyncWriter::write_all_at

https://github.com/neondatabase/neon/pull/10063#discussion_r1880439513

Signed-off-by: Yuchen Liang <yuchen@neon.tech>

pageserver/src/virtual_file.rs

@@ -1332,8 +1332,8 @@ impl OwnedAsyncWriter for VirtualFile {
         ctx: &RequestContext,
     ) -> std::io::Result<FullSlice<Buf>> {
         let (buf, res) = VirtualFile::write_all_at(self, buf, offset, ctx).await;
-        let x = res.map(|_| buf).unwrap();
-        Ok(x)
+        res?;
+        Ok(buf)
     }
 }