plugin NAT Port: fix errors handling:

`miniupnpc` raises `Exception("Success")` when no device is found
(instead of returning 0), which was resulting in an exception breaking workflows in
Libervia.

The code now catches unexpected exceptions, and handles `Exception("Success")` as "no
device found".

libervia/backend/plugins/plugin_misc_nat_port.py

@@ -91,16 +91,23 @@ class NatPort(object):
         self._upnp = None
 
     def _discover(self):
-        devices = self._upnp.discover()
-        if devices:
+        try:
+            devices = self._upnp.discover()
+            if not devices:
+                raise exceptions.NotFound()
             log.info("{nb} UPnP-IGD device(s) found".format(nb=devices))
-        else:
+            self._upnp.selectigd()
+            self._external_ip = self._upnp.externalipaddress()
+        except exceptions.NotFound:
             log.info("Can't find UPnP-IGD device on the local network")
             raise failure.Failure(exceptions.NotFound())
-        self._upnp.selectigd()
-        try:
-            self._external_ip = self._upnp.externalipaddress()
-        except Exception:
+        except Exception as e:
+            # the miniupnpc binding raises Exception("Success") when no device is found,
+            # instead of returning 0
+            if str(e) == "Success":
+                log.info("Can't find UPnP-IGD device on the local network")
+                raise failure.Failure(exceptions.NotFound())
+            log.warning(f"UPnP-IGD discovery error: {e!r}")
             raise failure.Failure(exceptions.FeatureNotFound())
 
     def get_ip(self, local=False):