proxy: report blame for passthrough disconnect io errors (#8170)

## Problem

Hard to debug the disconnection reason currently.

## Summary of changes

Keep track of error-direction, and therefore error source (client vs
compute) during passthrough.

proxy/src/serverless/websocket.rs

@@ -1,3 +1,4 @@
+use crate::proxy::ErrorSource;
 use crate::{
     cancellation::CancellationHandlerMain,
     config::ProxyConfig,
@@ -7,6 +8,7 @@ use crate::{
     proxy::{handle_client, ClientMode},
     rate_limiter::EndpointRateLimiter,
 };
+use anyhow::Context as _;
 use bytes::{Buf, BufMut, Bytes, BytesMut};
 use framed_websockets::{Frame, OpCode, WebSocketServer};
 use futures::{Sink, Stream};
@@ -165,7 +167,11 @@ pub async fn serve_websocket(
         Ok(Some(p)) => {
             ctx.set_success();
             ctx.log_connect();
-            p.proxy_pass().await
+            match p.proxy_pass().await {
+                Ok(()) => Ok(()),
+                Err(ErrorSource::Client(err)) => Err(err).context("client"),
+                Err(ErrorSource::Compute(err)) => Err(err).context("compute"),
+            }
         }
     }
 }