set marks and pop OpenSSL errors until
sets a mark on the current topmost error record if there is one.
will pop the top of the error stack until a mark is found. The mark is then
removed. If there is no mark, the whole stack is removed.
ERR_set_mark() returns 0 if the error
stack is empty, otherwise 1.
ERR_pop_to_mark() returns 0 if there was
no mark in the error stack, which implies that the stack became empty,
ERR_pop_to_mark() first appeared in OpenSSL 0.9.8
and have been available since OpenBSD 4.5.