rbd.cc: propagate some errors to user-space when they're available

There was a bunch of situations in which we would have a proper error to
propagate to user-space but we would always return '1' (EXIT_FAILURE).

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>
(cherry picked from commit 7e722245a717038c49f432ae19d9874c25066bb6)