This should now show the error. Part of the problem was that NpgSqlException was inheriting from System.ApplicationException when it should inherit from System.Common.DBException. Now that it has the correct base class the message is shown as with MS SQL.
To remove the Admin Modules you may need to uncheck the "Exclude From Feature List" then save, then you can delete the module instance from Content Manager and then finally delete the module.
Thanks,
Joe