OSD: tell them they died if they don't exist as well

OSDMap::get_down_at() asserts that the osd exists.

Fixes: #5223
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
OSD: *inodes_hard_limit must be less than the fd limit

Also add a comment explaining that.

Fixes: #5224
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>