As I recall, the NCR Century 200 computer I worked on had a row of bulbs (not LEDs) on the front panel. It think there were around 16, and the operator manual stated that if there was an error, the computer would stop and the error code could be determined by examining the "number" in the bit patterns of the lights.
Thankfully my employer had also invested in a state-of-the-art teletype (the sort that you put a typewriter ribbon in, and you put fanfold paper through with holes punched along the side), and the teletype would helpfully spit out a more informative message, like:
5100 INOPERATIVE
Although you usually could operate "by ear" as if the job was printing a huge number of statements on the line printer (the fast printer), which made a terrific racket, and the printer suddenly stopped, you would then hear the teletype pathetically typing away in the background "5-1-0-0-I-N-O-P-E-R-A-T-I-V-E" which took about 5 seconds.