Currently, in the case a filesystem goes read only will crash with a "can't take write lock" message.

It would be preferable for the following:

1. Mongod should continuously monitor the state of the file system where /data is located for being in read-only state
2. When read-only state is detected mongod should immediately step down as a master and exit gracefully
3. If mongod is started while the file system is in read-only state mongod should block and wait until the file system becomes writable; in this blocked state it should be seen as "recovering" by its peers much like the state when mongod is replaying journal files after restart.