issue #19143: more helpful error message if username isnt in instance db

The way this thrown error gets reported up to the client is a bit crazy. But I've seen this before and I don't know if we have a way to throw such an error neatly. See how it includes the word throw? It also doesn't evaluate the string if you put "username " + XT.username + " etc.". It just copies that line of code and sends *that* to the client.

XT.Response [database error]: undefined() LINE 30:       throw "No result for locale. Username probably does not exist in the ins