Always return real WAL restore error

If we return the context error it will be "context canceled" but the
errgroup has the real error. Context should never be canceled unless the
errgroup had errored.

replica.go

@@ -1233,12 +1233,14 @@ func (r *Replica) Restore(ctx context.Context, opt RestoreOptions) (err error) {
 	}
 
 	// Apply WAL files in order as they are ready.
+applyFor:
 	for index := minWALIndex; index <= maxWALIndex; index++ {
 		// Wait until next WAL file is ready to apply.
 		mu.Lock()
 		for !walStates[index-minWALIndex].ready {
 			if err := ctx.Err(); err != nil {
-				return err
+				mu.Unlock()
+				break applyFor
 			}
 			cond.Wait()
 		}