msg_socket: return io:Error from pair() instead of Option
msg_socket: return io:Error from pair() instead of Option

Change-Id: I8733794bca7a9510d5508941cbe3297a78e9923b
Reviewed-on: https://chromium-review.googlesource.com/1314210
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Tested-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm