IO Thread Reports False Error When Stopped During Connecting to Primary

If quickly running

start slave io_thread;

stop slave io_thread;

and the IO thread is stopped during its "handshake" with the primary (i.e during safe_connect() or get_master_version_and_clock()), it will report IO Errno: 2013 in SHOW SLAVE STATUS.

However, this isn't actually an error, as the user aborted the IO thread. The code should be updated to not report an error when the connection is killed due to user intervention.