OSD/OSDService: make osdmap load error more human-understandable

OSD/OSDService: make osdmap load error more human-understandable

In case when OSD crashed leaving empty OSD maps in its store, subsequent
OSD boots will fail with exception end_of_buffer thrown in map->decode.
This occurs because _get_map_bl returns TRUE and empty bufferlist.
In that case, inform user of actual issue instead of crashing in odd place
and requiring them to strace ceph-osd to find out what actually happened.

Signed-off-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>