pageserver: spurious basebackup error on timeline shutdown

[jcsp]

ERROR page_service_conn_main{peer_addr=123.123.123.123:53454}: query handler for 'basebackup XXX YYY --gzip' failed: timeline shutting down

This is incrementing basebackup failure counter in a way that can trigger alerts.

Need to track down what code path is responsible for this, and tweak error propagation so that we don't count timeline shutdowns toward our error counter for basebackup.

[arssher]

This presumably should fix it
#11046