Catch unknown sync exceptions when opening device and show error.

Some devices lack writable info and gpodder assumes they are writable.
This can lead to an exception being thrown when not writable.

src/gpodder/syncui.py

@@ -105,14 +105,22 @@ class gPodderSyncUI(object):
                 done_callback()
             return
 
-        if not device.open():
+        try:
+            if not device.open():
+                self._show_message_cannot_open()
+                if done_callback:
+                    done_callback()
+                return
+            else:
+                # Only set if device is configured and opened successfully
+                self.device = device
+        except Exception as err:
+            logger.error('opening destination %s failed with %s',
+                device.destination.get_uri(), err.message)
             self._show_message_cannot_open()
             if done_callback:
                 done_callback()
             return
-        else:
-            # Only set if device is configured and opened successfully
-            self.device = device
 
         if episodes is None:
             force_played = False