crimson/osd/pg_recovery: fix exception handler scope in recover_missing

Member access binds tighter than ?:, so the handler attached only to the
recover_object() branch and exceptions from recover_object_with_throttle()
escaped unhandled. Parenthesize the ternary to cover both branches.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>