correctly detect that gtkspell is not installed

src/features_window.py

@@ -209,7 +209,7 @@ class FeaturesWindow:
     def speller_available(self):
         try:
             __import__('gtkspell')
-        except ImportError:
+        except ValueError:
             return False
         return True
 

src/gui_interface.py

@@ -3022,7 +3022,7 @@ class Interface:
             try:
                 import gtkspell
                 spell = gtkspell.Spell(tv, lang)
-            except (ImportError, TypeError, RuntimeError, OSError):
+            except (ImportError, TypeError, RuntimeError, OSError, ValueError):
                 dialogs.AspellDictError(lang)
 
         if gajim.config.get('soundplayer') == '':