pbrd: don't silently fail on atomic match IP change attempts

pbrd: don't silently fail on atomic match IP change attempts

Currently pbrd does not support the abilitity to make atomic
changes to a config.

ex)
`match src-ip 1.1.1.1/32`
`match src-ip 1.1.1.0/24`

We would overwrite the first one but never actually install it.

In the `set nexthop commands` we explicitly fail if there is
already a `set nexthop` config present. This patch extends the
match src/dest-ip configs to do the same.

In the future we should make all these commands atomic but for
now its better to not fail silently at the very least.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>