Improve error handling in LSM merge finalizing.
Panic in LSM if a merge or switch fails at the wrong time.

The wrong time is once changes have been made to the LSM tree structure
in the merge case, or any time during a switch.
Normalize the panic macros to match WT_ERR / WT_RET semantics, use them consistently.
Panic if we fail to unlock an LSM tree: something has gone badly wrong.
Merge pull request #1097 from wiredtiger/lsm-merge-error

Improve error handling in LSM merge finalizing.