runner: Send the right alert for handshake message parsing failures.

This throws me off every time.

Change-Id: I19848927fe821f7656dea0343361d70dae4007c9
Reviewed-on: https://boringssl-review.googlesource.com/23445
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/test/runner/conn.go

@@ -1383,7 +1383,7 @@ func (c *Conn) readHandshake() (interface{}, error) {
 	}
 
 	if !m.unmarshal(data) {
-		return nil, c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
+		return nil, c.in.setErrorLocked(c.sendAlert(alertDecodeError))
 	}
 	return m, nil
 }