Handling vim.error from omnifunc

If the omnifunc encounters a syntax error, we need to handle this without
showing a traceback to the user.

python/ycm/completers/all/omni_completer.py

@@ -95,7 +95,7 @@ class OmniCompleter( Completer ):
         raise TypeError( OMNIFUNC_NOT_LIST )
 
       return filter( bool, items )
-    except ( TypeError, ValueError ) as error:
+    except ( TypeError, ValueError, vim.error ) as error:
       vimsupport.PostVimMessage(
         OMNIFUNC_RETURNED_BAD_VALUE + ' ' + str( error ) )
       return []