pageserver: don't validate vectored get on shut-down (#7039)

## Problem
We attempted validation for cancelled errors under the assumption that
if vectored get fails, sequential get will too.
That's not right 100% of times though because sequential get may have
the values cached and slip them through
even when shutting down.

## Summary of changes
Don't validate if either search impl failed due to tenant shutdown.

pageserver/src/tenant/timeline.rs

@@ -863,8 +863,6 @@ impl Timeline {
         fn errors_match(lhs: &GetVectoredError, rhs: &GetVectoredError) -> bool {
             use GetVectoredError::*;
             match (lhs, rhs) {
-                (Cancelled, Cancelled) => true,
-                (_, Cancelled) => true,
                 (Oversized(l), Oversized(r)) => l == r,
                 (InvalidLsn(l), InvalidLsn(r)) => l == r,
                 (MissingKey(l), MissingKey(r)) => l == r,
@@ -875,6 +873,8 @@ impl Timeline {
         }
 
         match (&sequential_res, vectored_res) {
+            (Err(GetVectoredError::Cancelled), _) => {},
+            (_, Err(GetVectoredError::Cancelled)) => {},
             (Err(seq_err), Ok(_)) => {
                 panic!(concat!("Sequential get failed with {}, but vectored get did not",
                                " - keyspace={:?} lsn={}"),