Fix ParseHeader() can only report the last file's error in non unityBuild mode

[zhaojunmeng]

        if (i < Headers.size() - 1)
            // Before modified, the ParseResult is just deleted without checking whether it has any error.
            delete parser.Parse({ Headers[i] });
        else
            res = parser.Parse({ Headers[i] });

Change to: if any file has non success ParseResult, just stop parsing and return the result.

[zhaojunmeng]

Error parsing 'Builtins.h, Classes.h, Classes2.h, Delegates.h, Enums.h, Overloads.h'
/home/runner/work/CppSharp/CppSharp/tests/Builtins.h(3,10): fatal: 'cstddef' file not found
CppSharp has encountered an error while parsing code.

It seems that the error that already existed in the previous CI pipeline has been exposed by this merge quest.😭