mon: DataHealthService: shutdown mon if failed to obtain disk stats

Being unable to run a ::statfs() may be a symptom of something bigger.

We want to cleanly shutdown the monitor ASAP if such thing happens.

Fixes: #4509

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: DataHealthService: log to derr instead if we're about to shutdown

Otherwise the message would -- or could -- be lost.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>