[orca] Add more error handling to navlist

[Joanmarie Diggs <joanied src gnome org>]

commit 79539f3782408cd506579a8fc2556c9a74660ce4
Author: Joanmarie Diggs <jdiggs igalia com>
Date:   Fri Nov 30 13:48:08 2018 -0500

    Add more error handling to navlist

 src/orca/orca_gui_navlist.py | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)
---
diff --git a/src/orca/orca_gui_navlist.py b/src/orca/orca_gui_navlist.py
index c2c79c5d8..176c6903e 100644
--- a/src/orca/orca_gui_navlist.py
+++ b/src/orca/orca_gui_navlist.py
@@ -150,26 +150,38 @@ class OrcaNavListGUI:
     def _onActivateClicked(self, widget):
         obj, offset = self._getSelectedAccessibleAndOffset()
         self._gui.destroy()
+        if not obj:
+            return
+
         self._script.utilities.setCaretPosition(obj, offset)
         try:
             action = obj.queryAction()
+        except NotImplementedError:
+            msg = "ERROR: Action interface not implemented for %s" % obj
+            debug.println(debug.LEVEL_INFO, msg, True)
         except:
-            debug.println(
-                debug.LEVEL_FINE, 'Could not perform action on %s' % obj)
+            msg = "ERROR: Exception getting action interface for %s" % obj
+            debug.println(debug.LEVEL_INFO, msg, True)
         else:
             action.doAction(0)
 
     def _getSelectedAccessibleAndOffset(self):
         if not self._tree:
-            return None
+            msg = "ERROR: Could not get navlist tree"
+            debug.println(debug.LEVEL_INFO, msg, True)
+            return None, -1
 
         selection = self._tree.get_selection()
         if not selection:
-            return None
+            msg = "ERROR: Could not get selection for navlist tree"
+            debug.println(debug.LEVEL_INFO, msg, True)
+            return None, -1
 
         model, paths = selection.get_selected_rows()
         if not paths:
-            return None
+            msg = "ERROR: Could not get paths for navlist tree"
+            debug.println(debug.LEVEL_INFO, msg, True)
+            return None, -1
 
         obj = model.get_value(model.get_iter(paths[0]), 0)
         offset = model.get_value(model.get_iter(paths[0]), 1)