miniupnpd: suppress grep and uci errors

miniupnpd: suppress grep and uci errors

If miniupnpd is installed but disabled or not running, the hotplug
script will query uci for keys that don't exist and grep a temporary
config file that doesn't exist, resulting in the following errors:

uci: Entry not found
grep: /var/etc/miniupnd.conf: No such file or directory

These would arise when an interface is brought up or down, and are
more confusing than helpful, especially when miniupnpd is disabled.

Suppress these errors.

Signed-off-by: David Ehrmann <redacted>