gnutls: Fix check for certain handshake failure conditions

It seems we have no tests to see if these errors work or not, because
the code is comparing a GTlsConnectionBaseStatus code to a GnuTLS error
code. It has been broken since 0e94c20a.
The code previously used "status" as the name for a GnuTLS error code,
then changed it to be a GTlsConnectionBaseStatus in that commit. Alas, I
should have used a different intermediary name instead, which would have
caught this as a compiler error.

Getting the tests to return particular errors is likely impractical.
Let's just fix this and move on.