mongo_lock's find_mutex_holder should handle mutexes with no apparent holder

In mongo_lock.py, find_mutex_holder() looks up a mutex holder in a thread_dict map, but doesn't check for the key's existence first.

If the key doesn't exist, the entire MongoDBDumpLocks run fails with a backtrace and this Python error message:

KeyError: 17150

Instead, it should simply print an informative message and continue.