Fix error msg for instances on offline nodes

Currently, for both primary and secondary offline nodes, we give the
same message:
- ERROR: instance instance14: instance lives on offline node(s) node3
- ERROR: instance instance15: instance lives on offline node(s) node3
- ERROR: instance instance16: instance lives on offline node(s) node3
- ERROR: instance instance17: instance lives on offline node(s) node3

This is confusing, as an offline primary is in a different category
than a secondary. The patch changes the warnings to have different
error messages:
- ERROR: instance instance14: instance has offline secondary node(s) node3
- ERROR: instance instance15: instance has offline secondary node(s) node3
- ERROR: instance instance16: instance lives on offline node node3
- ERROR: instance instance17: instance lives on offline node node3

Thanks to Alexander Schreiber <als@google.com> for reporting this
issue.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Alexander Schreiber <als@google.com>