What are error messages for?
> "Server Error 500."
ISTM there are two issues here.
The first is to appease irate users who have made the basic error of relying on technology that still needs another 5 minutes under the grill before it will be ready for everyday use by everyday users.
The second is that the function of an error message is to communicate state in an accurate (top priority) and concise form. Such that those in the position and with the ability to resolve it receive the information they require.
In the second case short numerical error messages do the job very well. Provided the error codes are unique and well enough defined that a single code points to a single cause of failure (and from there, hopefully a well defined fix). Their nature also makes it easy for a non-techy to report the exact message to those who need to know. Often without too much prompting.
However, they also provide a perfect platform for tech-haters to criticise systems for "gobbledygook" as their meaning is obscure to the average person. However, would a longer message, along the lines of "I'm so sorry but the computer is completely buggered up at the moment. We will be restoring normality just as soon as we are sure what is normal" be any more helpful than "Oooops, sorriy", or just :( ?