Journaler: pay attention to return codes from read head.
Journaler: pay attention to return codes from read head.

Previously we ignored them, except for printing them out. This could
lead to bad things like creating new journals for non-existent MDSes
if you entered an invalid rank during --reset-journal.

Also assert that the stripe unit is valid
before using it as  a divisor.

Signed-off-by: Greg Farnum <0bcb4295d38ee745130ea5f2bb88ef4eef474e61@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph