SSL socket error logging can be improved.

While debugging some unrelated linkage problem, I noticed the following error output;

16:19:25 I0728 16:19:25.323794 26188 libevent_ssl_socket.cpp:1244] Socket error: error:00000005:lib(0):func(0):DH lib

The error message appears not very helpful and that we can improve on.

When receiving a libevent openssl-error, we do not check the error code but pass it on to openssl for retrieving an error string – this is not ideal considering that openssl does signal more;
The error code 5, which actually means SSL_ERROR_SYSCALL does hint that we should now check errno for more information on the problem. We should only ever invoke openssl's error string generator when we did receive a SSL_ERROR_SSL.

Also see http://openssl.6102.n7.nabble.com/SSL-read-return-1-error-00000005-lib-0-func-0-DH-lib-tp27612p27613.html