liblog: improve errno handling on read failure

1. socket() failures should skip close(), since sock is not an open fd
2. on other I/O related failures, preserve errno around the close() call
3. set errno to a meaningful value on check_log_success() failure

Change-Id: I0fb9e726ae5fee2f7b84344aa925e5159d57c360
Signed-off-by: Greg Hackmann <ghackmann@google.com>