A fine idea.
You can add that by changing
line 379 in core/components/phpthumbof/model/phpthumbof/phpthumbof.class.php from
$this->modx->log(modX::LOG_LEVEL_ERROR,'[phpThumbOf] Could not generate thumbnail: '.$this->input.' - Debug: '.print_r($this->phpThumb->debugmessages,true));
to something like
$this->modx->log(modX::LOG_LEVEL_ERROR, '[phpThumbOf] Resource ' . $this->modx->resource->get('id') . ' :: Could not generate thumbnail: ' . $this->input . ' - Debug: ' . print_r($this->phpThumb->debugmessages, true));
Or you can try out
pThumb, my fork of phpThumbOf. I've just added this feature, since I agree it'd be quite useful sometimes. pThumb has a bunch more bug fixes and improvements as well.