[The Alchemist]

False negatives will be encountered in websites which have any error code as a default content like some programming forums.

I suggest you take initial page content and page content after adding apostrophe(final page) and then compare. Approach can be like counting number of errors in initial page and final page and if number of errors encountered are more in final page than in initial page then , it is vulnerable.

PM me if you want my help and we can make a better version

I'll keep this in mind too while making a later version of this Vulnerability Scanner.
Thanks.