audio filters: display input/output formats when conversion fail

src/audio_output/filters.c

@@ -186,7 +186,9 @@ int aout_FiltersCreatePipeline( aout_instance_t * p_aout,
     if ( !i_nb_conversions )
     {
         /* There was only one conversion to do, and we already failed. */
-        msg_Err( p_aout, "couldn't find a filter for the conversion" );
+        msg_Err( p_aout, "couldn't find a filter for the conversion "
+                "%4.4s -> %4.4s",
+                &p_input_format->i_format, &p_output_format->i_format );
         return -1;
     }