table: fix mishandled _sstable_deleted_gate exception in on_compaction_completion
table: fix mishandled _sstable_deleted_gate exception in on_compaction_completion

on_compaction_completion tries to handle a gate_closed_exception, but
with_gate() throws rather than creating an exceptional future, so
the extra handling is lost. This is relatively benign since it will
just fail the compaction, requiring that work to be redone later.

Fix by using the safer try_with_gate().

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla