auto-t: handle exceptions correctly for start_ap

The start_ap method was raising potential dbus errors before converting
them to an IWD error type. This is due to dbus.Set() not taking an error
handler. The only way to address this is to catch the error, convert it
and raise the converted error.

autotests/util/iwd.py

@@ -425,7 +425,11 @@ class Device(IWDDBusAbstract):
         self._wait_for_async_op()
 
     def start_ap(self, ssid, psk):
-        self._prop_proxy.Set(IWD_DEVICE_INTERFACE, 'Mode', 'ap')
+        try:
+            self._prop_proxy.Set(IWD_DEVICE_INTERFACE, 'Mode', 'ap')
+        except Exception as e:
+            raise _convert_dbus_ex(e)
+
         self._ap_iface = dbus.Interface(self._bus.get_object(IWD_SERVICE,
                                             self.device_path),
                                             IWD_AP_INTERFACE)