zebra: On Nexthop install failure don't set Installation failed

zebra: On Nexthop install failure don't set Installation failed

Currently FRR when installing a nexthop group, the installation can fail.
The assumption with the code was that the current nexthop group was
not already installed.  This leaves a problem state where if the
users of the nexthop group are removed, the nexthop group will be
removed possibly leaving a orphaned nexthop group in the data plane.

FRR on a nexthop group installation does not actually know the status
of the nexthop group in the kernel.  It's possible that a earlier
version of the nexthop group is left in play.  It's possible that
there is no nexthop group in the kernel at all.  Leaving the
Installed flag alone allows upon Zebra removing the nexthop
group when it is removed from zebra.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>