pageserver: revise logging of cancelled request results (#10604)

## Problem

When a client dropped before a request completed, and a handler returned
an ApiError, we would log that at error severity. That was excessive in
the case of a request erroring on a shutdown, and could cause test
flakes.

example:
https://neon-github-public-dev.s3.amazonaws.com/reports/main/13067651123/index.html#suites/ad9c266207b45eafe19909d1020dd987/6021ce86a0d72ae7/

```
Cancelled request finished with an error: ShuttingDown
```

## Summary of changes

- Log a different info-level on ShuttingDown and ResourceUnavailable API
errors from cancelled requests

pageserver/src/http/routes.rs

@@ -3393,7 +3393,17 @@ where
                             let status = response.status();
                             info!(%status, "Cancelled request finished successfully")
                         }
-                        Err(e) => error!("Cancelled request finished with an error: {e:?}"),
+                        Err(e) => match e {
+                            ApiError::ShuttingDown | ApiError::ResourceUnavailable(_) => {
+                                // Don't log this at error severity: they are normal during lifecycle of tenants/process
+                                info!("Cancelled request aborted for shutdown")
+                            }
+                            _ => {
+                                // Log these in a highly visible way, because we have no client to send the response to, but
+                                // would like to know that something went wrong.
+                                error!("Cancelled request finished with an error: {e:?}")
+                            }
+                        },
                     }
                 }
                 // only logging for cancelled panicked request handlers is the tracing_panic_hook,