Revert "aout: make log message to be debug as there isn't any real error if no filters needed"

If the user explicitly asks for a filter, it's an error to not be able
to instatiate it.
This reverts commit 7b0761c4.

src/audio_output/filters.c

@@ -498,7 +498,7 @@ int aout_FiltersNew (audio_output_t *aout,
                                          name == visual);
         if (filter == NULL)
         {
-            msg_Dbg (aout, "cannot add user filter %s (skipped)", name);
+            msg_Err (aout, "cannot add user filter %s (skipped)", name);
             continue;
         }