Improve error handling in LSM merge finalizing.

There is a related comment at `lsm_tree.c:784` in `__wt_lsm_tree_switch`:

 784 err:    /* TODO: mark lsm_tree bad on error(?) */

I think this dates from before WiredTiger had panic.  I'd suggest doing a panic in both places for now.  We can already panic in some other situations where writes fail.  If we figure out something more graceful in the future we can improve things, but a panic is better than continuing with data inconsistent between disk and memory.