watchfrr: Modify some stderr messages to zlog_warn

watchfrr: Modify some stderr messages to zlog_warn

The stderr output is not being displayed as part of watchfrr invocation
in system startup.  Specifically if the user has not properly sent
1 or more daemons to monitor.  If the end-user is using tools/frr
this stderr is dropped( and systemd appears to drop stderr too? )

Modify the two stderr calls in this situation and use the zlog system.
Now I can clearly see an error message that tells me what has gone wrong.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
[DL: fixed typo]