Manual:$wgShowExceptionDetails

Details
If set to true, uncaught exceptions will print a complete stack trace to output. This should only be used for debugging, as it may reveal private information in function parameters due to PHP's backtrace formatting.

With the default false, users are shown only an error message "Fatal exception of type MWException" with an alphanumeric code that a person with access to the server logs can use to find the stack trace. Without this information, it's impossible to help fix the error.