fix: restore not logging ignored io errors as errors (#4120)

the fix is rather indirect due to the accidental applying of too much
`anyhow`: if handle_pagerequests returns a `QueryError` it will now be
bubbled up as-is `QueryError`. `QueryError` allows the inner
`std::io::Error` to be inspected and thus we can filter certain error
kinds which are perfectly normal without a huge log message.

for a very long time (b2f5102) the errors were converted to `anyhow` by
mistake which made this difficult or impossible, even though from the
types it would *appear* that we propagate wrapped `std::io::Error`s and
can filter them.

Fixes #4113, most likely filters some other errors as well.

pageserver/src/page_service.rs

@@ -352,7 +352,7 @@ impl PageServerHandler {
         tenant_id: TenantId,
         timeline_id: TimelineId,
         ctx: RequestContext,
-    ) -> anyhow::Result<()>
+    ) -> Result<(), QueryError>
     where
         IO: AsyncRead + AsyncWrite + Send + Sync + Unpin,
     {
@@ -398,7 +398,9 @@ impl PageServerHandler {
                 Some(FeMessage::CopyData(bytes)) => bytes,
                 Some(FeMessage::Terminate) => break,
                 Some(m) => {
-                    anyhow::bail!("unexpected message: {m:?} during COPY");
+                    return Err(QueryError::Other(anyhow::anyhow!(
+                        "unexpected message: {m:?} during COPY"
+                    )));
                 }
                 None => break, // client disconnected
             };