vtls: don't use OpenSSL's ERR_PACK.

[agl]

ERR_PACK is an internal detail of OpenSSL. Also, when using it, a
function name must be specified which is overly specific: the test will
break whenever OpenSSL internally change things so that a different
function creates the error.

[mention-bot]

@agl, thanks for your PR! By analyzing the history of the files in this pull request, we identified @rousskov, @bagder and @ghedo to be potential reviewers.

[bagder]

thanks!