openssl: Introduce openssl_perror().

openssl: Introduce openssl_perror().

Openssl has a decent error reporting framework, but we only employ
it if get_random_bytes_or_die() fails. This patch abstracts out a
new helper which prints the error string of the earliest error code
from the thread's error queue. We make the helper return -E_OPENSSL
unconditionally as this simplifies callers a bit.

Only get_random_bytes_or_die() calls the new helper for now but
additional callers will be added in subsequent commits.