Always salvage WiredTiger metadata regardless of error code when starting with repair

Currently, we only attempt to salvage WiredTiger metadata when the return code from wiredtiger_open is WT_TRY_SALVAGE. Unfortunately returning this error code from all points of failure in WiredTiger is difficult. There are cases, like WT-4345, where WT_NOTFOUND is returned instead, but an unconditional salvage would have allowed --repair to succeed.