When reloading a watched file fails the apm reports that in the error log, so if you are tail'ing the log you should be able to notice it.

I agree with Randy that an error in a reloaded file shouldn't be able to bring down the entire site. A visual indication of a failure would be nice, but to be honest for my personal use it's sufficient to know that the error log needs to be inspected when reloading didn't bring the desired results.