I think you should start with the "Reboot" part below and then maybe try the "Failing install" if that doesnt work.
Reboot - If you've added the SQL install as a pre-requisite, it should have an Options section in the Properties tab of that pre-requisite with the option to "Reboot system after installing this prerequisite".
Failing install - If you've added the SQL install as a pre-requisite, it should have a Main Setup File section in the Setup Files tab, provide a property for "Save return value in this property" and you should be able to have a custom action check that property for the return code. This should be done alongside Properties -> Options -> "Continue with main installation even if the prerequisite is not installed" being checked, as you will now have to check manually.