Go back to my original idea of checking for EILSEQ and EINVAL and failing

Go back to my original idea of checking for EILSEQ and EINVAL and failing
immediately if either of those is set after wxVsnprintf call returns a
negative number to indicate some type of failure.

All other errno are assumed to simply mean that the buffer is undersized so
the platform C library function can set errno to anything it likes or simply
not set it at all and our code will keep trying to increase the buffer size
just as it did originally.

This should (hopefully) fix compilation with C libraries that only support
the most basic set of errors (of which EINVAL and EILSEQ are part).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@49342 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775