$wgShowExceptionDetails is configuration setting which shows more detailed output about error which happens.

Default value is false, so 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.

I think to we should change default value to true, as usually we suggest users to do when report some problem, so always user can get detailed information about error.