DeviceMgr: nicer message when pairing

The experience is a bit better if the user is prompted to connect,
instead of showing an error.

lib/plugins.py

@@ -472,7 +472,7 @@ class DeviceMgr(ThreadJob, PrintError):
             infos = self.unpaired_device_infos(handler, plugin, devices)
             if infos:
                 break
-            msg = _('Could not connect to your %s.  Verify the cable is '
+            msg = _('Please insert your %s.  Verify the cable is '
                     'connected and that no other application is using it.\n\n'
                     'Try to connect again?') % plugin.device
             if not handler.yes_no_question(msg):